In the unlikely case of somebody getting interested in what I'm saying, I refer to
this. You can google more yourselves, this type of information is luckily available for general public.
Please note that in the linked article, the personnel are pilots. However, they were helicopter pilots, who also do not encounter high long term G-forces. Granted there is always turbulence, but for example the USAF AN/AVS-6 binocular weight is around 590 grams. Monocular device is probably around half of it, and that's what the ground forces typically use. The cause of neck problems typically is the uninterrupted contraction of the back of the neck muscles who are continuously working against the one-sided load. This in the long term, can lead to misalignment of vertebra.
Yeah it's not an apples to apples comparison and so on, and I don't believe that there is a significant cause of concern, but I do believe this is something that Rift users should at least be aware of. The weight of the thing is close to those things that reportedly do cause problems, and its center of gravity is close to those devices which have caused problems.
And yes, I have carried an NVG myself.