The main driver of motion sickness in VR is "moving without moving", where your virtual viewpoint moves independently of your actual head position and confuses your brain. That's a common issue in VR games where practical space limitations mean that some kind of artificial locomotion is usually required, but Apple seems to be purposely avoiding that kind of experience by keeping the viewpoint locked to your head at all times.

Third party immersive apps might break that rule, of course.