Quoting: TheRiddickHere is some games with issues to illustrate my point. MESA/RADV, no idea about nvidia.

ELEX (requires workaround or gfx glitching)
Risen3 (same as above but crashes randomly often)
Max Payne 3 (tessellation bugged on RADV)
Operation Flashpoint 2 + Red River (needs gfw fix, but menu ctd, doesn't render).
NMS (people complain about random issues all the time)

The list goes on, basically Valve needs to address some backward compatibility with older games via proton more IMO.

Then we have all the anticheat stuff on top... for the most part broken with proton.