I can’t say more than anecdotal, but after a broken neck and back (shoulder blades and up), I cannot sleep on my stomach at all. I haven’t been fused or anything, but I lost around 1/2-1/3rd of my neck rotation to the left.

Sleeping on my stomach used to be a thing, but now it will leave me in terrible shape for days. The deviation in alignment of the spine is more significant than it seems. When your range of mobility is reduced significantly, the effect is far more noticeable as abnormal. It is about like falling asleep in a yoga pose. Those are some of the most fatigued muscles in the body, just to hold up your head. Damage some and it becomes extremely evident.