

I didn't play long, but seemed to work fine

Depends on your configuration, so give it a try!
Some text in menus may not appear until you hover them with the mouse. Most background textures flicker while playing a level. Past that, the game remains completely playable and enjoyable.
Whether you will be able to run this game with Proton depends heavily on your configuration; my two older laptops could not run it at all, but this one does. If the game stops at launch with an error 1009, there isn’t anything you can do.
There is however a C++ port that should run with most Proton configurations: in the Steam properties of the game, switch to the cpp_test beta. It’s a few updates behind and doesn’t have editor/community levels support, but you’ll be able to enjoy the main story, all official levels, and some Hellgate levels.

Out of the box, PulseAudio has too poor latency to enjoy this game. I followed the instructions on the Arch wiki (https://wiki.archlinux.org/index.php/PulseAudio/Troubleshooting#Setting_the_default_fragment_number_and_buffer_size_in_PulseAudio) to set my card latency down to about 15ms and now it's playable, and additionally installed the lowlatency kernel in the Ubuntu repos because hey it can't hurt. Things could still be better, but audio latency on Linux is a huge can of worms and I could easily spend weeks tuning and never actually play the damn game!

Runs into a couple ActionScript errors. The first one is "ArgumentError: Error #3759: The selected texture format is not valid in this profile." The second is "TypeError: Error #1009: Cannot access a property or method of a null object reference."