I think more often enemies should use the environment against the PC's. We are infiltrating their lairs, fighting on their turf, after all. But I would REALLY hate it if they dropped a boulder on me and my Rogue would not get a Dex save to evade.
I don't know if it's a new thing, or I just never saw/experience it before but couple days ago when attacking goblin encountment they rolled a massive boulder on Laez:
Yeah, I agree that there should be a defencive check for the receiver of the enviromental damange. But Larian gonna Larian, and keep adding guaranteed sources of damange. That sounds like something that could be potentially moddable though.