In general, I would say capacity feels alright (turning armor to shields to 0% MASSIVELY reduces how strong the shield feels, except in special cases that I'll talk about too). I think fast regen and low delay is king in SB considering it's usually pretty easy to avoid hits for a couple seconds (MUCH easier than in Borderlands) so perhaps shields should be generally skewed to have longer recharge delays and slightly slower recharge rates. Capacity could honestly even be increased a little to compensate in this case, but that's just my thought.

I said it in the comments already, but amp/roid/turtle shields generally feel really bad. The Bee is, naturally, insane, and I think Hide of Terramorphous feels really good for a roid shield, but non-uniques of these types feel pretty horrible. Turtle shields especially, as the shield capacity available in SB is not even close to what is achievable in Borderlands, so the tradeoff simply isn't worth it. I would probably increase the capacity AND reduce the hp penalty on these because all they are ultimately are hefty shields. Compare these to adaptive shields, which have decent capacity but BUFF your max hp (sometimes very significantly) AND give you a big resist bonus! Turtle just feels bad. (it'd go away from BL balancing, but Turtle giving some additional armor-to-shield could go a long way to making it feel good, too)

Looking forward to the nova/spike/turret changes- I've tested them out and am not a fan, but they don't seem actively BAD, just not really my style. If I had to offer some feedback, I'd say nova is tough to use if you're not melee (but probably intended anyways); turret never seemed to do much (bullets might be too slow? tooltip damage seems very good but I never noticed the turrets killing stuff). Turrets could go the BL3 route and be mobile/airborne, which would go a LONG way to making them powerful. Spike is generally pretty good and Vlad is actually the one I'm hunting right now because I think it feels really good.

Overall, I'd say uniques are too good. While I recognize the "vanilla" shields aren't meant to be utilized as more than stepping stones, once you get even a t4 unique shield that fits your build it's pretty much better than t10 purples. Out of the ones I've found and tested, Evolution (adaptive) feels INCREDIBLY strong, to the point where my character had >1000 hp. Tortoise feels awful for obvious reasons but I think the whole shield is a joke anyways so idk about that. Many of the others felt in-line in comparison with Hide of Terramorphous feeling great and Vlad the Impaler also being really smooth. I haven't done a lot of farming for uniques so those are the only ones I've really used for more than a minute or two to test (and I haven't found most uniques yet anyways).

Looking forward to the next version!