I've worked with VR helmets since the 1980's in flight simulation.
The problem is simple: Your eyes use two mechanisms to figure out distance - the degree to which your eyes have to point in different directions in order to fuse two images into one - and the degree to which the lens has to be stretched or squished to pull things into focus. Every VR helmet ever made gets the first thing right - and completely fails at the second thing. No matter what optics are used, no matter anything - you're focussing at the same distance over the entire visual field, regardless of virtual distance.
When our brains look at two inputs that should yield the same results - but they don't - we assume that something is malfunctioning, and we get sick.
Same deal with seasickness when the inner ear says one thing about the motion and our eyes tell us something different.
So - you need some kind of insane computer-driven lenticular display where every pixel has a lens that focuses that light at an appropriate depth for the 3D content at that point. Such things don't exist...and that's the only thing that'll make this problem go away.
All of the recent people to try to fix this are amateurs who just started looking at it - look back at the research done by the old flight simulation companies like Link, Singer and Rediffusion - and the decades of research on this subject done by AFRL (the Air Force Research Labs), the US Navy and NASA.
WIthout solving the focus problem, we're doomed to another cycle of dizzy, puking customers.
Worst of all - US Navy research shows that after a protracted time in one of these VR rigs, it's dangerous to go out and drive a car or fly a plane - their pilots aren't allowed to fly within 24 hours of being in a simulator.
-- Steve