Potential problems I see are
1) They are designed for impacts on solid surfaces, not water. That could be a problem.
2) A restriction in the jumper's ability to turn his/her head
3) Immobilization of the neck in an impact might make lower sections of the spine more vulnerable to serious injury
4) Lack of movement in the neck could mean that more force is transferred to the head, i.e., a negligible reduction in neck injuries could lead to a significant increase in concussions. (although I would take a concussion over a neck fracture anyday)
5) If there is a real benefit, would it be at all levels, or only for advanced jumpers?
I am no authority on this subject, so I have no answers. I do think , however, that I have come up with some legitimate questions. If other sports with similar impact speeds are using these neck braces, it seems to me jumpers should be, at least, looking into it. Anything short of not skiing in the first place that makes this
safer sounds like a great idea to me.