Started playing Underrail a few days ago (had tried the alpha a few months ago but didn't really get into it then), and wanted to make a few comments about psionics based on my experiences so far. Logged about 25hours according to Steam so far, which includes trying several different builds all the way to getting to Junkyard.
First, I have no experience of the old shroomhead feat, but the new one is worthless imo - I would not take this for any character, as ultimately it translates to receiving approx 75 credits per mindshroom picked up (using 700 credits cost of psi booster / 170MP x 15 psi per mindshroom picked up). I've no issue with the feat being as it is for flavour purposes, but as a selectable feat it looks extremely suboptimal to me.
Having said that, I encountered no difficulties whatsoever with a recent build that was pure psionic - skills were the 3 psionic disciplines, stealth, hacking, lockpicking, dodge and evasion (I wanted to try out a completely pure psionic user, so no other weapon skills whatsoever). Stats were str 4, dex 4, agi 7, con 6, per 5, wil 10, int 4. I played on classic xp (purely to test out the difference, as my first playthroughs were all using the oddity system). By the time I'd finished the outpost quest and the hopper quest, I had enough credits from sold equipment to purchase a galvanic vest(! - if I hadn't bought this, I could have purchased another 15-20psi boosters), 6 or 7 psi boosters plus a few other bits and pieces. I also found that despite having 0 in guns, I was able to equip 5mm and 7.6mm pistols which provided cheap attacks for when fighting easy or almost dead mobs.
I got through the entire GMS quest by using up all but one of the psi boosters I had, but of course once you've finished that quest you have a lot of loot that can then be sold for more psi boosters. I had enough credits under the new economy system to pick up all the spells I could learn at that stage, and had a few nice guns and other loot left over to sell at Junkyard when I got there, plus a purchase of another 10 or so psi boosters.
I'm really not seeing any difficulty playing the early game as a pure psionics user, provided you make use of the starting pistol with zero skill invested for very easy fights where you really don't need to burn MP. I suspect that once you've finished GMS, the cost of buying psi boosters is no longer a real issue, so the fact that it was straightforward to do so tells me that the economy isn't overly unforgiving even for a pure psionic build.
The only caveat to the pure build in my mind is how the early game plays on oddity - I intend to restart under that option to compare the two xp systems, but I don't think this will materially change the difficulty.
