I'm a side sleeper with an "ultra firm" mattress and soft mattresses mess up my whole alignment. I never get sore shoulders or hips, even if I sleep all day like I recently did. I can also reliably switch to back sleeping, which I usually do when I'm sick because of congestion.
It's all anecdata, but I think people are just different.
It's all anecdata, but I think people are just different.