Major assumptions

What are these major assumptions everyone is talking about? I don’t wanna sound ungreatful, cuz I think the UE4 is a giant leap in the empowerment of the little guy… BUT. I stumbled over this:

this is from 2006, (prey) on the id Tech 4 (Doom3) and everything I want from the UE4.

So again, what are the assumptions and why are they preventing us from having a fixed timestep physics calculation and a gravity VECTOR instead of a float right out of the box?