I just loaded a save and repeated an earlier snakeman battle (just to be sure it wasn't a fluke from this one battle), and one of my pawns being sprayed took acid damage to the pelvis, ribcage, torso, and leg from the same spray. It's also definitely the case that some of my own characters take significantly less damage maybe because of either good "armor" or being behind cover, she took only 24% buildup. Over her torso, she was wearing a duster with 60% burn resist over armor with 30% burn resist and a shirt with 20% burn resist, taking only about 6 total damage in the initial spray, but every hit from the acid build-up still does 5 damage, where armor seems to matter less if at all. After the two acid build-up ticks, she wound up taking 15 total damage. A broader issue may be that these continuing acid damage burns can hit the toes or other places that armor doesn't cover in the base game.
She marks the first person I've seen actually not get downed by a spray because she took so little of it, and the continuing damage only hit her torso so she "only" took 28% pain from the mildest acid injury I've seen.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3466849810
The later burns were to the left arm (2.6 damage, 5% pain) and the torso burn jumped from 1.5 damage to 8.4 damage with a second burn to the torso, adding another 13% pain.

The pain buildup still nearly guarantees a down - a colonist who took 19% pain from a single constrictor tail hit (and 4% pain from scars) was downed two continuing acid burns after the snakemen were already retreating because of 91% pain from an acid splash (not counting other factors) that got him after having 50% acid build up (I didn't even see when it happened, and it wasn't reported in the log, so it surprised me he was even hit. However, he was at 60% pain after the first) Here he is with one continuing acid burn:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3466846902
Two continuing acid burns later, he's downed:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3466847214
He eventually hit 119% total pain, of which 96% was from a single splash that got him to 50% build-up.

I have a record of the snakeman that took a direct hit from friendly fire that I'm going to put into its own post because I took more detailed notes on him...

One friendly fire incident from point-blank range sprayed another adderman. It certainly seems like you can get hit more than once per spray, and this guy took 206% acid buildup from being immediately next to the sprayer.

Again, this attack seems to operate like a flamethrower attack and can hit more than once, where flamethrowers mention they attack in a "burst" of 10. Hence, saying that it does ~20+30 damage isn't really accurate, it can potentially do 200+300 damage.

Further, testing with creatures of different sizes implies that acid damage is scaled to size (which is kind of a big deal to the Big and Small mod series.) The acid spray is therefore unusually potent against larger creatures.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3466846145
Immediate damage was 10 to the sternum (23% pain), 5 to the left lung (12% pain), 15 to the torso (35% pain), and 25 to the snake body (59% pain) for instant down. (129% pain from the initial spray, before any continuing acid damage. This is also 55 initial damage from just the spray itself, before continuing acid damage from build-up plays out.) There are no other wounds (besides a 4.8 damage old stab scar to the left arm with no pain), just being caught in one friendly fire acid splash.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3466850298
The initial acid spray specifically hit the left lungs and sternum, as I've seen in a couple other acid burns. Part of the apparent variation in how much damage is reported may just be that the game only reports one "burn" per body part, so multiple aicd hits to the same location can stack without it being apparent how many burns are stacking, making documentation require memorizing/recording the state of the burns and seeing what changes...

Also, if it matters, this is a bodysize 0.8 snakeman.

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3466883100
After 10 continuing acid build-up burns (100% of the 206% build-up), he stands at 40.8 damage to the snake body (98% pain - and a real showing of why this body part needs armor, if only racial "tough skin" armor for this one part like a tortoise shell if it is meant to take the most damage), 18.9 damage to the torso (44% pain), 9.8 damage to the sternum (23% pain), 8 damage to the right arm (19% pain), left lung 4.9 damage (11% pain), left arm 7.9 damage (19% pain), snake spine 3.9 damage (9% pain). 223% total pain, 94.2 total damage. The damage has already started to slightly heal, resulting in the loss of a percent of pain here or there and a bunch of decimals.

I will point out I specifically saw damage to the snake body spine from the continuing damage: https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3466846521

After 15 continuing acid burns, he had 52.6 damage to the snake body, and was at only 3/56 part health left. https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3466898902
After 16 continuing acid burns, the friendly fire victim suddenly expired, exact cause unknown. The last burn appears to have been 4 burn damage to the tail, although that by itself should not have been fatal, and there was no bleeding. I can only guess there's some sort of cap on total damage a body can suffer before there's a forced death.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3466899532

I didn't do a tally on overall pain just before death, but overall damage appears to be 52.6 snake body, 18.9 torso, 9.8 sternum, 4.0 tail, 11.9 right arm, 4.9 left lung, 11.8 burn to left arm (plus 4.8 from an old stab scar), and 3.8 snake body spine damage. Total 122.5 damage at time of death.

EDIT: Actually this seems to be death by massive damage. A size 1.0 pawn dies if they take 150 damage, and it's proportional to body size, so a size 0.8 pawn will die at 120 total damage (not counting missing limbs). Hence, this was a straight-up one-hit kill from a single sticky acid splash, and no treatment besides possible amputation would have helped.

Going back to the later raid's save, things obviously played out a little differently playing it again. I nevertheless managed to get some friendly fire acid sprays and some of my pet velociraptors (ReGrowth: Extinct Animals mod) sprayed...

To make sure I keep accurate notes, since the information disappears in the game, I'm going to just put all the data into this post, even if it gets a bit monotonous. Consider it !!SCIENCE!!

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3466968903

The velociraptor (Bella, I name by alphabetical order starting with the animal's name) takes 160% acid build-up from a point-blank shot. 3 immediate splash hits (neck, body, right leg) for 6.7 damage each, and 14% pain each. (Total 20 damage and 42% pain.)
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3466969719
Bella is a juvenile velociraptor that is only body size 0.38. Bella continues to engage with the adderman that was also splashed and then another and takes other wounds (worth 37% pain, and including a punch to the jaw for 6.8 damage) before collapsing in pain at 85% pain after taking the first acid tick for 3.2 damage to the right foot for 7% pain.

Second acid burn does 3.2 damage, 7% to the left leg. Third acid burn to the right foot again, 3.2 -> 6.4 damage, 7% -> 13% pain. Fourth burn to left foot 3.2 damage, 7% pain. Fifth burn to the left leg 3.2->6.4 damage, 7%->13% pain. Sixth to the left claw (which apparently feels pain just like other parts) for 3.2 damage, 7% pain. Seventh burns the jaw, which takes it off entirely because the part only had 9 HP, but this means the jaw doesn't hurt anymore. Eighth burn right leg 6.7->9.9, 14%->21% pain. Ninth burn neck 6.7->9.9, 14%->21% pain. Tenth right claw 3.2, 7% pain. Eleventh neck (separate because the old wounds to neck and right leg were tended) 3.2, 7% pain. Twelfth right leg (new burn besides tended) 3.2, 7% pain. Thirteenth to left leg 6.4->9.6, 13%->20% pain. Fourteenth right leg again, 3.2->6.4, 7%->13% pain. Fifteenth right foot 6.4->9.6, 13%->20% pain. Sixteenth and final burn to the right foot 9.6->12.8, 20%->27% pain. 160% acid build-up technically survived, but with no jaw, the velociraptor has 0% manipulation and is only good as cannon fodder or sammich meat. 70.3 total burn damage. 140% pain from burns ignoring the 7% pain that would have come from the burn to the jaw that melted off, while only 16% pain from non-acid wounds.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3467077562

The adderman takes a less direct blast, and "only" has 75% acid build-up.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3466970397
Once again, the left lung in particular is struck by the initial acid splash. (I haven't seen most other non-bone organs hit with the acid, just the left lung and bones, so unless I'm just rolling funny, there may be something about specifically the left lung being considered "exterior") Also, ignore that it shows the pelvis taking damage there, that's actually the snake body spine. (I have to use substitutes to fit a serpentine body to the Nice Health Tab mod's model, and there is no pelvis on a serpentman, and no snake body spine icon, so I swapped that in.) The initial splash causes 5 torso damage (12% pain), 10 snake body damage (23% pain), and 5 damage to the left lung (12% pain). Total 20 damage, 46% pain from the initial splash.

A bite and a gunshot after the initial acid damage for 22% and 19% pain each put the adderman at 86% pain and downs him. The first continuing acid damage tick does 4 damage, 9% pain to the right shoulder. Second further damages the snake body 10->14 (23%->33% pain). Third acid hits the jaw 4 damage, 9% pain. Fourth acid hit burns a wooden hand for 4 damage and no pain for obvious reasons. Fifth takes the snake body 14->18 (33%->42% pain). Sixth also to the snake body 18->22 (42%->52% pain). Seventh also to the snake body, 22->26 (52%->61%). The eighth and final burn (depleting all 75% build-up) for this adderman is to the left arm, 4 damage, 9% pain. Total burn damage at the time of the last build-up burn was 51.6, including 4 to a wooden hand. (Some small amount of damage healed by the time the acid ran its course, and that also lowered pain by a few %.) Total pain from acid damage 110% (no pain from the wooden hand), with 31% pain from non-acid wounds.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3467078132

Finally, as the serpentmen start to rertreat, another velociraptor (Walkyrie) takes three hits with an "acidic attack" from bites for 154% acid build-up. I wasn't aware the bites themselves did acid damage until this point.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3466986952
The tail and claw were already destroyed as old wounds. The left leg bruise is 11.6 damage and 10% pain. The left eye immediate bite/acid damage/scar is 14.2 (of 15 max HP) and 6% pain (which seems low for an eye injury...), 14.3 immediate bite/acid damage to the body with 18% pain, 18 bite/acid damage to the left foot for 23% pain. 46.5 damage caused by the immediate acid splash. 57% total pain, 47% caused by the immediate acidic attacks.

Another hit to this velociraptor adds more acid build-up. For a second, she's at 207% acid build-up as she takes 13.4 damage to the right leg and 17% more pain from this. (72% total pain)
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3467008838
This velociraptor is a cycle of burns behind the others. (All continuing acid burns resolve simultaneously.) The neck takes the first acid build-up tick for 5.6 damage, 7% pain. The second is the jaw, 5.6 burn, 7% pain. This second continuing burn brings the velociraptor to 86% pain and she is downed with pain shock. Third is the body (separate from the acidic attack damage, unlike other burns that just stack) for 5.6 damage, 7% pain. Fourth hits the right claw for 5.6 damage, 7% pain. Fifth is right leg (also separate from the "acidic attack" damage) for 5.6 damage, 7% pain. Sixth hits the head 5.6 damage, 7% pain. Seventh to jaw 5.6->11.3, 7%->14% pain. Eighth left foot 5.6, 7% pain. Ninth to left leg 5.6, 7% pain. Tenth neck 5.4->11.0, 7%->14% pain. Eleventh jaw burned off at 15 damage, no pain. Twelfth right claw burned off at 11 damage, no pain. Thirteenth left leg 5.6->10.9, 7%->14% pain. Fourteenth right eye 5.6, 7% pain. Fifteenth left foot 5.6->11.3, 7%->14% pain. Sixteenth neck 11.0->16.6, 14%->21%. Seventeenth right leg 5.6->11.1, 7%->14%. Eighteenth left leg 10.9->16.5, 14%->21%. Nineteenth body 5.6->11.3, 7%->14%. Twentieth and final to neck 16.6->22.2, 21%->28% pain. Total 116.7 damage from acid build-up burns, 54.7 from four acidic attacks, 10.8 from a constrictor tail, for a grand total of 182.2 damage. Total 112% pain from acid build-up burns, although there is 35% burn pain "missing" from burning off smaller appendages, 60% from acidic attack burns, 9% from a constrictor tail bruise, for a grand total of 181% pain (216% if the appendages weren't burned off.)
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3467085703

My doctor treats Walkyrie Velo as they take acid damage without medicine. (She has industrial medicine, but the velociraptor is set for herbal only.) She does not go for the acid build-up, but instead goes for the initial acid attack damage first, starting with the left foot then body. (During the time of the second and third continuing acid burns.) Left leg is treated just before the fourth acid burn kicks in. I switch over to Bella Velo at this point, and stop the lost tail from bleeding. Another tend treats neck and right leg burns, but then the eleventh burn count hits and does more neck damage, which doesn't stack because one was tended and the other wasn't. Since tending existing burns is higher priority than tending the acid build-up, this basically guarantees you can't just tend through all the burns and get to the acid build-up with tending alone.

Specifically watching and taking notes shows that it seems like the damage and pain inflicted by the acid build-up is highly consistent once they start on a single target, although the adderman wears clothing too poor at protecting from heat (and velociraptors wear nothing at all) to discern how much armor might help from this time I watched the burns play out. Also, different targets seem to take more damage, with the smaller juvenile velociraptor apparently taking less damage per tick than the adult one. Since armor definitely didn't play a part on unarmored animals, I'm guessing the damage is scaled to body size?

Beyond that, the left lung in particular is a magnet for burn damage. However, I don't see the animals taking acid burns to organs or bones, only the humanoids do that, but it happens even to baseliners, although the snakemen have it happen as well.

A more general conclusion is that the continuing damage tends to do extreme amounts of pain. It only takes 80% pain to get to pain shock and downing, and each 10% of acid build-up tends to get you 6-10% pain, with acid sprays having a highly variable 24-206% build-up depending on where you're standing, and doing between 13% and 129% initial pain, all but the most glancing of sprays will down a pawn. In the "real" save I played through, I had a pawn I was actively treating for her wounds downed through pain shock from a 30% acid build-up and no other wounds besides the acid spray. The best method I've found for dealing with these is to just make peace with their faction so I can get something easier raiding me like mechanoids or sieges. Otherwise, the best defense is letting turrets and other pain-immune combatants like mechanoids or pawns with every limb a bionic soak the damage, or be ready to offer up sacrificial pets, because when you're outnumbered 5-to-1, you can't afford an unavoidable attack that is a guaranteed pawn down.

Pythonmen by comparison are a total cakewalk, taking 10% pain from a constrictor tail is nothing compared to the acid spray. Addermen are entirely disproportionate in their likelihood to down a target. I haven't cracked open the mod to check, if these are being valued similarly in terms of raid value, they really shouldn't be, as I've taken a total of 10 damage from a pythonman, while I've had hundreds from addermen. The defoliator and mech cluster I had before and after the latter snakeman raid were significantly easier than the snakeman raid, so I again have to say you're seriously undervaluing addermen. I really have to stress I'm getting hit with these on day 66 (10 snakemen) and day 107 (24 snakemen), and they're coming in roughly triple the numbers of other raids while I still don't have my base defenses really built or key research like gun turrets complete. (I used turrets stolen from raider camps.)

Was comming for a bug, got a book.

Just wanted to throw in that I caught screenshots of the "direct hit to the brain" in the wild, so there's definitely problems with direct attacks doing damage to internal organs not exposed. ("I've heard of having an open mind before, but THIS is ridiculous!")
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3510170752
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3510171480
A "forcibly befriended" VIPER commando (amusingly, from the last one of these "right on top of you" raids...) stands in the doorway and launches sticky acid at the neanderthal behind the scrap metal from the drop pod, and a character in the "splash zone" takes a direct shot to the brain. (6.7 damage to the brain/skull/head, the brain only had 6 HP because she had the "dwarfism" trait. Happy is a faun from Roo's Faun Xenotypes if you're wondering. She wore no head armor.) The stomach and torso also took 1 damage, with her wearing a hyperweave jumpsuit (94% heat armor) and recon armor (60% heat armor.) Strangely, despite being an imperial, she had no death acidifier, but I think this might be because she's a faun and it interrupts the nature connection mechanic? (Woohoo! Excellent zeushammer delivery!)

The actual target, Synadenos, took 4.3 initial burn damage to the right arm, another 4.3 to the right leg, and 1.7 to the liver/torso. 28% acid build-up from this shot at maximum range. His arm heat armor only comes from a flak jacket (normal) for 10%, his leg armor only comes from cloth jeans (normal) for 6%, but the torso damage went through the flak jacket (10%), flak vest (27%), and a shirt (3.6%). (That should be 63% of the acid getting through, so it was about 2.7 initial splash damage to the torso before being reduced by heat armor.) Assuming the pre-armor damage was 4.5 and 4.7 to the arm and legs, that's a total 11.9 pre-armor acid spray damage spread to three locations.

Using another character to acid spray again, I manage to line up a fairly decent spray that hits several enemies at once. (Yes, I paused the game and left for several hours before coming back to the same fight to progress a couple seconds to hyper-analyze this. What?!)

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3510435017
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=3510435329

"Synadenos" from above went from 18% acid build-up to 71% acid build-up. (+53%.) His right leg is now at 16.1 damage and he took 5.8 damage to the right tibia, implying a total of three burns to the leg in total. (I'm guessing 6 damage from the acid build-up hitting the leg, 5.8 from the new acid initial damage, and 4.3 from the previous post's injury.) This acid spray, in spite of being aimed past him, clearly hit him more directly, but he took the least damage, regardless.

"Areksuma" is downed with 90% acid build-up from 10 damage to the left leg, 10 damage to the left lung (but not the torso, oddly), and 10 damage to the left clavical and shoulder. Downing came from damage to the legs and loss of consciousness causing movement to drop to 5%. His armor is a similar to Synadenos with flak jacket, flak vest, cloth clothes underneath, all normal quality. His downing is partly because of taking a shot to either leg already, putting the legs in orange and red damage.

"Kourkouas" takes 90% acid build-up from one hit, 10 damage to the spine/torso and 10 damage to the left leg. She's wearing recon armor (46% heat armor) and cloth underneath.

"Isidorus" takes 60% acid build-up but no internal organ damage, just 10 damage to the left arm and 10 to the torso.

*EDIT: Areksuma died of extreme total damage later while trying to treat his wounds. The others all died on downing anyway.