Perhaps it has to do with the optical center being offset? I don't have one to test it out, but if it works like it says online, the optical center would move f

Perhaps it has to do with the optical center being offset? I don't have one to test it out, but if it works like it says online, the optical center would move from somewhere in the middle of your eye to somewhere in the middle of the device, which would cause changes in parallax, and therefore mismatch in movement between the inner ear and what is inferred from the eyes. Maybe that's what causes the discomfort.