

Reduced graphics settings.
Certain text is very small and there's no scale setting in-game. I've heard you can increase UI scale via .ini edits but haven't tried myself -- it's only an occasional issue.
Controller support is spotty in that it's missing a few UI elements from XCOM 2; most notably, during combat there's no icon highlights for your selected action. You just have to read the tooltip display and infer the cursor's position along the row of actions.
Aside from that small gripe, it's a great fit for SD!

cap to 40 fps
Previously game wouldn't work due to 2k launcher stealing focus on Steam Deck. After a late 4/22 Steam Deck update, the problem is fixed.

Steam Deck. I have tried 5+ older versions of Proton. Proton 6.3-8 seems like it works right up until the main game menu loads, then the 2k launcher pops back on top. I have tried the command line option shown in the earlier comments to skip the launcher, but this prevents the game from loading at all with an instant crash back to steam.