There seems to be a lot of misconception to how much work the PhysX engine actually does behind the scenes in Unreal. It’s not just the vehicle simulations or whatever, it handles ALL of the collision code for the entire game whether an object is simulating physics (use that term loosely) or not, it calls back into the line/ray tracing code, it handles the destruction etc.
A lot of the other physics engines being posted are much more specific. Bullet is fairly feature complete, but I wouldn’t say it’s fast nor easy to use.
My only potential complaint, is that Complex Collision in UE4 is pretty slow. A couple of hundred skeletal meshes that are simulating physics and have complex collision bring my framerate down to around 30 in an empty level. I believe there’s room for improvements on that part.