The majority of popular PvP shooters use anti-cheat which does not work on Proton, so "almost all modern games" seems like overselling it to me.

But the stuff that does work, works well. I play Helldivers 2 via Proton on Fedora, and i experience far fewer crashes and instances of weird behaviour than friends on Windows or Xbox.

ARC Raiders is currently all working perfectly for me. Such a blessing. I hope it stays this way.