Another poster mentioned more conservative defaults, which certainly doesn’t help compatibility.
There’re also issues with any non-free software that might be a dependency of the game you want to run.
And finally Debian has a focus on stability, so it takes much more time for software updates to filter their way through the debian ecosystem before they’re released.
Roll all of that together and you’ve got a system that’s anywhere up to a year or two behind the released versions of things your games need to run, and isn’t necessarily motivated to improve the situation.
Flatpak Steam and similar systems should be mostly fine, until you need a fix that’s just recently been rolled into the Linux kernel or your DE or your GPU software stack. If you want the most compatible gaming system you really want to chase current releases of everything in the kernel/library/DE/GPU driver stack and that’s just not feasible on Debian unless you’re building a ton of your own packages on top of Sid. I did that for a while and eventually just switched to Arch instead.
SeramisV