Without Proton, the game still runs natively, but only in a low resolution (800x600) and I could not alt-tab. I could not even force a window change through switching workspaces (couldn't switch workspaces either). Secondary monitor was completely disabled on running, and once game was closed, primary monitor was stuck in 800x600 resolution. Had to change display settings and reset to defaults to revert back to normal. Once Proton is used those issues were fixed.

Mouse would constantly escape the game window even when fullscreen; i.e. mouse was not confined to game window boundaries, when running with Proton. Would constantly escape the game. Had to use a workaround to force secondary monitor in a diagonal space to primary monitor and simply be careful not to escape the game in the corner of the screen.

Some areas of some custom maps resulted in massive frame loss, until the entire map was cached, I guess. It corrected itself over time. Large numbers of custom enemies also resulted in some slow down, but that's the risk of joining massively modded servers.

Sometimes custom maps and resources would download at a rate much slower than nominal. Disconnecting and reconnecting fixed the issue. An issue from native KF is that if a game ends and a map change occurs while you are in the process of downloading resources you will suffer "connection failure." Annoying, but just reconnect.