I've experimented with our own game engine and VR sickness. When the view doesn't accelerate relative to your real-world position, there is no issue. The problem starts when you're using external direct movement control, such as a mouse and keyboard. Moving forward and suddenly starting to move backwards is slightly uncomfortable. Rotating your head while rotating the mouse in the same direction is the worst for me.
It's the inverse of car sickness, and affects some people but not others. My coworker has no issue using full FPS mouse/kb controls within VR.
It's the inverse of car sickness, and affects some people but not others. My coworker has no issue using full FPS mouse/kb controls within VR.