I just wonder, because even when they "set" the focus of these HMDs to "infinity", your eyes are still technically only looking at a screen at a fixed distance, mere inches away. Everything I have read about so-called "simulator sickness" and other maladies in HMDs have said that eye fatigue results from this "fixed focus" issue. It doesn't cause as much issue for pilots training in simulators, because the actual screen being projected on is several feet away from their eyes, allowing them to become more naturally focused toward "infinity" (parallel).
I also wonder about the head and neck areas and fatigue. Most of the HMDs - while lightweight compared to systems of the 1990s - still seem like they have a lot of "nose weight", and aren't balanced on the head. Proper balance (fore/aft and laterally) is crucial to long-term use with lower fatigue, even with a light weight HMD. Also, the more weight that rests on the bridge of the nose, the worse the amount of fatigue.
Much of this knowledge is based on research that was done in the 1990s and earlier, both with those early HMD systems, and other earlier simulator systems. I'm continually surprised that current commercial offerings seems to either be unaware of such research, or that they ignore it for the sake of marketing (distributing the weight on the head results in a more "helmet like" HMD, which probably isn't as saleable to the public; ultimately, the public probably would like to see something as lightweight and wearable like a pair of sunglasses, but current display and optics technology isn't there yet - if it ever will be).