

Everything works normally, I haven't encountered any problems whatsoever.
Works out of the Box
Runs natively. Install and play

Opted into the "open beta" patch 2.0.4 (still the most up-to-date version, as of the time of writing) which prevents launching the game through Steam on Linux natively, due to a configuration error. A solution is to launch the game outside of Steam, directly from the executable: "ImperatorRome/binaries/imperator". Either terminal or file manager works. Steam still recognizes the game as running.
Straight forward setup and overall stable
Crashed only twice and never again since
It did crash twice while using the invictus mod , but never on the base game. But it only crashed those two times even with the mods, and never again since. The crash did show the paradox crash report screen, not sure if they were caused by some proton issue or just some weird unique bug from mods alone.
Aside from that no problems whatsoever! Just set your Proton version (8.0-2) , install and play.
Good game but abandoned
It was ok
The top left banner was glitched
This game has terrible stutters that makes it unplayable. Removing your acess to /dev/input with "sudo chmod o-rx /dev/input" works as a workaround, but it can break some stuff. You can revert it by running "sudo chmod o+rx /dev/input"
Recommend increasing the UI interface scaling to 70%. Text is occasionally small but still readable.
Runs fine out of the box
The game had problems in the past seems that they are all fixed by now, at least on an AMD GPU.
Severe stuttering issues every 3-4 seconds make the game extremely unenjoyable to play. No solution currently exists to my knowledge.
Severe stutter, practically a pause, every 3-4 seconds
For those having trouble with the launcher, make sure your graphics card drivers are updated and that you have the dependencies. My game was working flawlessly until I ran apt upgrade and then it stopped launching. Didn't work as native, with any version of Proton, or direct launch from command line. Manually updating my amdgpu drivers did the trick. There are several threads on steam regarding glibc and adding launch parameters as well.

2.0 : Game won't launch on Proton and have game-breaking bugs with native
On native a bug make levies only get one type of unit which can make the game almost unplayable for some cultures (hellenic will only get archer until you unlock legion, forcing you to rely on mercenaries).
Couldn't get the game working with any version of proton (3.X, 4.X, 5.X, 6.1-GE-2).
The Steam launcher doesn't work. I launched the native version through the terminal and it works perfectly

Doesn't launch natively or with Proton
Proton (any version) won't launch the game. If the native version is launched on Steam, Steam shows the game as running, but the game itself doesn't launch.

Couldn't play with 5 versions of Proton, or natively.
I was able to get the launcher to work with v4.2-9, but versions v5.0-9, v4.11-13, v3.16-9 and v3.7-8 didn't load. In all cases I couldn't launch the game (tried different resolutions / full / windowed), or it crashed the launcher.
Didn't got any issue until my game crashed after 4 hours. Didn't tried multiplayer.
Just got one crash in 4 hours
I have many graphic issues (water not rendering etc) on native and game was not starting with proton, since the last hotfix (1.4.1, released 2nd of April) game launch properly (just need to be a bit patient as it take some times to load).
Works fine but needs much more time to start than the native Linux version
The first Loading screen takes long and freezes sometimes
If I quit to the main menu
Before the recent patch(1.4 “Archimedes”) I couldn't start the game at all. Now I can run the native version however in the native linux version there is a bug that all ocean water is not displayed and the small threes on the map but if I run it with wine(or proton in specific) I can see the ocean water so it is fine

Game freezes/crashes in menu
Since the native Linux version is completely broken for me(none of the available fixes worked for me) I tried to run it with proton but the game freezes in the menu.

Does not work with native and proton
Native launches the launcher, but then crashes when entering the game, and the proton launches the game but when you select a new game or tutorial it freezes when "loading saves"
Works flawless. This is ProtonDB: the proton version runs fantastic! ;) Native is borked somehow, at least on my machine.

Native and Proton Failed
Tried Native, Proton 4.11-9, 4.2-9 and 3.16-9.
Native: Doesn't run, just crashes immediately.
Proton: Game loads, menu appears, then when entering Single Player Tutorial or New Game the loading indicator stops at "loading savegame" and the computer becomes unresponsive.
Tried turning Steam Overlay off, same behaviour.
Does not work native, you'll need proton

Linux version fails to launch (borked) playing native. Works great under Proton, no issues.