It doesn't seem that ambitious to me really. It's almost like taking GTA:SA making it bigger, forcing a FPP and giving it a better AI and giving it a change of setting. GTA:SA worked fine, why not STALKER?
It's the AI that's the issue, I believe; from what I gather as well as the expected intelligent fighting behaviour (flanking, taking cover, all that rubbish), they're trying to get all the NPCs to function as if they were entities in an actual ecosystem; hunting, hiding, sleeping, etc.
AFAIK GTA:SA never had that sort of independent interaction between NPCs; they'd meet, travel about, but they wouldn't actually go and search out stuff to do, for example.
I'd imagine it's an absolute bastard to test a system designed to simulate individual interactions, because it's potentially completely unpredictable.