I can kinda see where you get at, but there is some discrepancy between our opinions.
T1 and T2 can more easily reach veteran upon building completion and level faster than Elite as well. As such, they have the edge when it comes to mods anyway.

Mods are the deciding factor for me. A T4 unit without a mod is often weaker than a T2 with one. They are gamechangers, capable of turning your chumps into deadly troops.
Most of my armies are composed of T2 units. Outlier are basically only Syndicate, where I form Indentured Overseer groups (though I heard that the Vanguard Scout Tactic is supposed to be pretty powerful too, the lacking mods early one probably make it worse than in Vanilla).

And thats where I see the problems. I basically never use Core units outside of Indentured. Core units are just that much worse on average (I suppose the Vanguard ones are not too bad, though).

So, IMO the core units would be the only ones that could be having an additional mod slot early on. A mod slot IS powerful, though.... Very.


Regarding the way the mod slots are reworked:
From my understanding, the mods have basically a list of units that can use these (or the units have lists of mods they can use). So, each 'normal' mod is tripled. They each disabled all other mods in the same 'tier'.
Basic units only have access to the first list, veterans have access to the first and second ones, and the final has all three. That is also the reason you see the mods tripled in the mod selection. In other words, to remove the mod restriction needs a bigger incension, since you need to rework the units (most likely). So, removing that has a bigger impact than what you would think at first glance.

Interesting. Thank you for the detailed answer, also regarding the more technical aspects on the slot limitation.

I completely agree with you regarding the relevance of unit mods. Modding a unit in a certain way helps it to be more effective, counter an enemy’s strength, or get more survivability. There is thus a certain strategical consideration or reasoning involved. As in, right now at this moment during the game I face a certain opposition of type X; unit mods are then simply a potential solution that I can use to engage this opposition and overcome it.

In addition to this pure strategic reasoning, I think Planetfall is fun because it encourages so much tinkering with your units. If you have researched the needed technologies and field a sufficient economy, you can really engineer the heck out of your units. Maybe it’s not the most efficient way to build your military, but for me it certainly is really fun. This ability to tinker is at least different in Adventure Mod, in comparison with the vanilla game.

As a side note, I like to play on smaller maps with slower research. As a consequence, T1 and T2 units stay with me for the better part of a playthrough. I tend to recruit T3 and T4 as well, but the main bulk is still T1 and T2. Maybe this is why I’m missing additional mod slots, especially very early in the game.

My main take away, however, is based on what you said in the beginning. Maybe T1 and T2 are indeed leveling fast-ish, thus unlocking a second or even third slot quite early. I have to pay closer attention to this. Thanks.

I am still wondering though about the motivation, in terms of game design, to limit the slot availability ...

The idea behind the limitation was to make units less of a throw-away commodity, and reward holding onto units more.
Basically, the evolution abilities are nice, but they aren't exactly gamechangers. The additional mod slots, however, are.

I suppose DerMentat wanted to prevent leveled units to be vastly more powerful than unleveled, while still make the incentitive higher to level your troops due to mod restrictions. Note that this also means neutral units are stronger, since they don't have such a restriction and can have up to 3 mods. Otherwise roaming armies would be (even more) of a joke, because your leveled unit has the same amount of mods (but better by default, because you can actually plan around it) AND have evolution abilities/stats that vastly outclass the roaming ones, expanding the gap even more.

I personally don't mind the system itself. It was annoying at the beginning, but I have learned to plan my army around it. It however did little impact me, except focusing on levels far more than before.
Most of the times, 2 Mod slots are the game changers (Lighting stun and arc, Psyonic panic and catharsis, ...). The fact that you can more easily reach the 'Champion' rank when building for non-elite units only reinforced my previous tactics.
I actually rarely field T3 or T4. Sometimes, when I play specific races/techs with certain layouts, I will field a pletora of T3 units (I love the Psy-fish ones for example), but most of the times it is neither necessary nor rentable.

A group of 6 Veteran Syndicate Sniper with Stun and Chain can easily decimate most non-stun immunes, even if they consist of T3 and T4 units, especially if you actually have 5 Sniper with a Hero in the mix. Heroes are ludicrious powerful, they often rival T4 units while also buffing the whole army and having diverse abilities and effects.
To begin with, there are only few T4 units I actually like. Reavers for example, but while Reavers are very powerful, so are Reverse Engineers. And you can often build two of them for one T4, etc... The Syndicate T4 unit, though? I really dislike. It feels so underwhelming to me...

This actually makes a lot of sense, the two-mod combination you mention. Now that I think about it, my shtick often involves a two-mod combination. Like Trenchers with Fortification Tools combined with Guardian Daemon. Nasty little buggers. I love them so much.

I share your feeling on the Zenith. In addition, the Subjugator is also somewhat disappointing. However, the Wraith is pretty amazing, not only because of its tactical potential but also because of its movement on the strategic map. Syndicate has also some sweets mods to put on the Wraith, granting extra teleportation range for instance.

I feel a bit more accepting of Adventure Mod’s slot limitation now, thanks. If only it was compatible with Expanded Arsenal though ...