Auto manufacturers spend a lot on time and money making sure as few people as possible will get injured using their cars. (with the exception the Pinto of course) There is a lot of strong structure that is added to the rear of the seat (actually the whole seat was redesigned) when they intended to add a head rest.
Looking at the design of these it looks like there would be a real possibility of the two legs, that are located right behind your back, would angle forward (at the base) as your head was thrown back in a rear end collision.
The force of these two, what I would think are two metal bars or some such, would probably make two holes in you back and probably make two large holes in your lungs. In this scenario you would not be getting your next, and last, ride with the siren playing and lights flashing. No need for them to rush....😢
I remember seeing those coin operated machines in the men's room at truck stops. There was a sign on the machine that said:
"Just for show not for go" The same words should be on these also..
And no, they will not prevent you from getting whiplash.