The way foam mattresses work is weight deformation. If your butt is heavier than your head, it sinks in more.
I’ve never seen any evidence to suggest this was good for spinal alignment. If you’re a side sleeper, it can also be bad for your shoulders if it doesn’t sink in enough.
My take is that this is good (for me). At least the body can sink.
If you don't sink evenly, your spine is aligned though not level. Whereas in a traditional mattress a poor fit to your body will feel like a hammock, or a hard floor with pressure points
I’ve never seen any evidence to suggest this was good for spinal alignment. If you’re a side sleeper, it can also be bad for your shoulders if it doesn’t sink in enough.