I've actually atempted most playstyles Styg. I only haven't tried mellee nor pure tanking. I don't like mellee because it predispositions me to take a lot of damage and I haven't tried pure tanking because getting a really good armor will take a while and if I'm tanking the damage, then I'm not going to invest into stealth as it will become nullified so it's hurting a lot at the beggining. It's not saying it's impossible of course, I just don't like things that are not as effective as others from the beggining.
Psi does scales well and possibly in the final build it will scale even above weapons with enough levels but it doesn't deals a big amount of damage. Yes it's AoE damage compensates for that along with it's CC capabillities. But it lacks in damage in general and my Psikers start at 10 will and get +1 every 4 levels as well as getting the psi-skills pumped up to the max every single level. They are no mk5 grenades levels of damage, they are no snipers or even assault guns in auto levels of damage. Again this is no complain because the level of utillity Psi has more than covers for it. What I said about it is. It's balanced, it's by no means OP.
I don't even know how is it possible to fire 3 specials bolts in one round of combat with the crossbow. Psikers is what I've used most extensively and I always use a crossbow to complement them. Now most crossbows cost 25 points per shot, with special tactics which I do tend to pick up they don't go higher than that. Still all characters have 50 action points so at best you fire 2 special bolts. Now I've seen crossbows which actually cost 21 AP to fire, they deal less damage in return which balances it out. Still to fire 3 special bolts per round or indeed to simply fire the crossbow 3 times in a round you'd need a crossbow that cost at best 16 AP to fire... This if I'm not missing anything, but I don't think I am. Now I never noticed whether or not they have a cooldown because it's incredibly rare for me to have to resort to special bolts. I never used more than one per round, In fact I don't think I ever needed to fire more than once per combat and it's rare the combat where I even need to use them. They would be there more for the lack of psi energy or for some reason inabillity to use Psi abillities and need to either stun or just pump electrical damage.
I don't like any CC outside Psi abillities because it's either extremelly limited in comparison or unreliable... I never said flashbangs were bad, all that I said is that by themselfs alone I don't consider good enough. Sure you can re-stealth and give them time to cooldown but that just seams a way to actually cheat the system by continuously enter and leave combat (provided you have enough grenades to do it, but as they will surelly become craftable in the future, it is something that will happen).
I always deal with raw numbers, I don't like unreliabillity. If you tell me that every attack has 80% chance to imcapacitate someone. Yeah I like it. If you tell me 15%... Meh. The approuch to combat in Underrail as I belive you have done is that you need to plan very carefully your approuch and then excute it in order to succeed. It becomes hard to plan an approuch if what you can do is unreliable at best.
There is a difference between investing into a skill that is required for you to use in combat and a skill in which you invest some point with the only purpose to get a psi abillity and then stop, especially if you don't intend to use psi-abillities at all except for that... It's kinda like, waste 30 or 35 (not sure the right value right now) skillpoints just to get 3 range to your mellee. But that is the way this feels to me because skill points are really about just enough to specialise, they don't allow for much diversity in my opinion. As for not increasing Psi abillities to see how effective my Psi abillities are... Well I know that they will start failing the CC not to mention they start losing the capacity to keep up with the damage you have to pump out. But a dedicated Psiker has no reason to do that and the thing is. So long as you keep your combat skills high, as you should, everything is close to 100% reliable.
I have given a lot of things in the game a fair chance, not everything as I said, I don't like mellee and tanking in this game because I feel that it the most unbalanced thing that needs work before it becomes enjoyable for me. But all non Psiker builds I've made I will tell that would have been a complete pain in the ass to play without mk5 and mk3. If I didn't had those I'd seriously reconsider plaing them. It's not that they wouldn't be able to still do things, but it would become so imbalanced to me that it would remove all the joy of playing them. Every fight would be a torment to find a way to kill everything without dying in the process.
I do know perfect balance is impossible, but at the end of the day so long as one build doesn't has it harder than other then that is balance. Obviously certain builds will struggle more against certain enemy types which is to be expected but that is ok. Obviously I don't mean to say all builds should be acceptable, of course not, bad builds should have it hard and people should learn to make builds in order to have a fair degree of success. That said mellee should not be weaker than anything else and so on.