I mean it's pretty obvious that MS made the Xbox One with compatibility in mind like Windows upgrades.
Sony obviously did not and figured "hey since PS4 and PS5 are essentially custom PC boxes, everything PS4 should just run on PS5, right?" and just like when you try to run 15 year old PC games on a modern super fast computer, a lot of times it just works fine, but some games don't and have issues. So Sony is going through and patching the issues one by one on a per game basis, but that takes a lot of time/manpower so good luck on all of them, they'd essentially be doing what GOG.com is doing.
that being said, I'd expect it to go like running most 2000-2010 era games on your PC right now. Most should run fine. I'd be surprised if the compatibility is < 75% of the entire library from launch.