I insta kill as much centaurs than alien, but for both of them, really far less than molerats. Also, since dying don't heal radiation and poisoning is much harsher than before, I think they're fine, noone is protected against nasty crit from them. The only real problem is dumb AI, but AI in 2238 have always been weird and I think it's a complicated thing.
Oh, and btw, I found far more interesting centaurs than aliens.