Another example of this is Left 4 Dead 2, ironically made by Valve. The Windows client is far better, but unfortunately Valve hasn't fixed a bug that prevents joining VAC-secure servers when playing on Proton. This means you need to find an "insecure" server or host your own game.

Civ 5 is another sad example. It has a native port by Aspyr, but it's clearly inferior to getting it to work with proton (and not just for lacking mods).