I was initially (with some unreliability) able to get my Steam Controller to work with the game. However, possibly after a game update, it no longer works and nor did my wired Xbox 360 controller. I was however able to get my 360 controller working once I disabled Steam Input. Credit to user "Crash Override" on ProtonDB for this workaround.
Graphical glitches, visible on Proton GE 6.15 (glowing contours after about 10 minutes play) can be fixed by forcing dx11 mode rendering. Add -dx11 -force to game's command line.
I just use -dx11, no -force, and I didn't have any graphical glitches. I'm using Mesa with AMD graphics, so YMMV
The graphical glitches with DX12 are also related to AMD and the ACO compiler, so that's an option as well. I wasn't aware that ACO worked fine with the DX11 shaders, thanks for letting me know.
I did need to switch to DX11 anyway, as a consistent spot in a specific cutscene always caused a VM fault in my RX 480 GPU, causing the system to become useless without a full reboot.
I'm having severe audio issues during the intro logo cutscene and the video playback in the intro seems unstable, with both dx11 and dx12. Happens with both Experimental and 6.3.
Psychonauts2-DX11.log
Psychonauts2-DX12.log
Log files in both DX11 and DX12 modes with Experimental
cant hold a key in psychonauts 2. It goes on repeat
Issue transferred from https://github.com/ValveSoftware/Proton/issues/5360.
@voidplayer posted on 2021-11-28T19:25:05:
You cant skip scenes with escape because it never hold the whole time. It shows like i were tapping the key
It happens the same thing with other buttons that need hold. Claravoyant, etc.. It works if you bound the power to second mouse button

Try to hold a key
cant hold a key in psychonauts 2. It goes on repeat
This has been fixed in latest update of the game
This has been fixed in latest update of the game
I saw an update yesterday, and it was fixed and i thought it worked already, but today its not working again
Compatibility Report
System Information
I confirm:
steam-607080.log
Symptoms
Media Foundation FMV content does not play. Game sometimes crashes switching languages or reloading from the menu after leaving a game to the main menu again, but this last one may be a symptom on Windows machines as well.
Reproduction
Start game with any official version of Proton, including Experimental.
Notes
The Media Foundation FMVs play just fine under the latest Proton-GE, 6.15 as of this writing, but that is unofficial. Proton-GE also has some graphical glitches in the game rendering that are not present in official Proton, so that's an issue I'll file with Glorious Eggroll, since it affects his runtime, but not the official.