

The inconsistent control issues would likely be too intrusive for the average player, unfortunately.
Sometimes would only respond to touch input, making it very difficult and/or inconsistent to play. Restart the game or Deck would sometimes fix the issue.
Not sure if the artifacts are Proton-specific.
Out of the box, the performance was significantly worse than on Windows for me. I can't recommend this one.
The characters on the battle animations were black squares.
The whole game was a bit sluggish. Loading times were slow and animations were slower than on Windows.
I got a crash with the message "Win32 function failed: HRESULT: 0x887 a0005 Call: GR_D3D_Device-> CreatureTexture2D at time 453 in file /TexturesM.cpp". I could solve it by using PROTON_USE_WINED3D=1 %command%, as a previous report suggested.
The battle animations were broken (characters and some background elements were replaced by black squares) and everything felt sluggish and laggy.
If you face this issue
Win32 function failed: HRESULT: 0x887 a0005 Call: GR_D3D_Device-> CreatureTexture2D at time 453 in file /TexturesM.cpp
It means issue with GPU setup.
Adding PROTON_USE_WINED3D=1 %command%
works for me.
Sometimes the controller is not recognized in the game despite being recognized by the system (is a DS4, I can use it to move the mouse)
Combat animations are significantly slower than on Windows
Sometimes the game freezes for around 30s, then continues working correctly

Worked super well out of the box for me, only minor issues that seemed intermittent
Fullscreen mode initially only took up the center of my 1440 monitor, but the issue went away for me after restarting game
Controller was occasionally not recognized. Can't tell if this is a game issue or compatibility though, sometimes hard to tell with these indie titles.
Fun knock-off Fire Emblem
Retaining the volume level settings between games seems a bit off. It can be quite loud, even if the sliders are set very low. Subwoofer owners, be advised.
Seriously though, if you have subwoofers: the SFX for blunt weapons is going to rattle your house.