My understanding is SteamOS was lacking hardware support for most devices, and that they’ve been slowly fixing this. It’s been mostly functional for awhile now, but used to be completely broken, and it’s only now that Valve is saying it’s actually ready for most AMD devices.

It’s part of why people had needed to use things like Bazzite/ChimeraOS on other gaming handhelds, because bare SteamOS wouldn’t work. There were things like hardcoded support for just the Van Gogh chipset, hardcoded TDP management that would cause system crashes on different hardware, no support for network devices, etc.