Anyone with at least 60 Biology can craft and use a Psi Reinvigorator and then a Psi Booster to clear the Inhibited status and restore 75 psi points, so there's an effective countermeasure that's not exactly costly in terms of required skill points. For those without psionics you can only try to run away, tank the damage (not really an option on Hard and above if there are other enemies, except maybe if you have 10+ CON) or rely on Dodge, which does very likely need heavy investment to be reliable. Yes, they are build-specific weaknesses which naturally should exist, but while you can outright eliminate the problem of being Inhibited, the damage from a doppelganger you can only attempt to mitigate. IMHO, that's not a small difference.
That's true, but for less than half that investment (and using much more readily-available component materials, too, which isn't a small difference - Statolith is not fun to farm) your non-psi user can create and pop morphine and take an advanced health hypo before it wears off. And on Hard or Dominating, eating the damage from a 300-skill Psicognitive Interruption (that's about the skill of a Goliath beetle with two friends nearby) is most certainly no less difficult than eating the damage from a doppelganger.
No, Hard is meant to be hard. I can't agree that this is an unbalanced scenario
if we give non-psi a fairly accessible way to handle doppelgangers. As it is right now, I'm sympathetic to the claims that doppelgangers are not fun or well-balanced. I don't agree with them, from my tanky sledge experience, but I'm also super nitpicky about action management and am willing to load a save if I die. I get that not everyone is.
crossfire's suggestion seems fun. What if psi doppelgangers couldn't be hurt by normal weapons but could be stunned for a turn by electricity? Everybody and their uncle carries a Taser anyway, right?