I'd say high G in maneuver makes HMD less useful to useless. HMD is heavier than conventional helmet because it moves lots of electronic circuit from the dashboard to the head. 100 grams is nothing when you expereince no extra G, but would be 600 grams if you are pulling a 6G turn which is quite frequent, it would be near a kilogram if you pull a 9G. This is enough to pin your head on the head-rest with only eyeballs movable. Since the advantage of HMD is the ability to sync the display with your head movement, without head moving HMD becomes meaningless. A HUD on the other hand has all information on the screen to see, you can just move your eye balls. A good example is rollercoaster raid, you should have experienced moments when you can not move your head, that is still low G compared to fighter jet.
HMD will always be heavier than conventional setup, the only thing one can do is to reduce that weight to a tolerable level under a limited G load (less intense combat).