Testing it myself, I found that XCOM 2 would only launch with certain mods enabled, and fail no matter which launcher I tried—both the old and new options provided by Steam and the Alternative ...

XCOM 2 for Steam is a special case, 2K said, as the launcher included Mod support functionality. Of note, these changes will let you enable mods on the Steam Deck.

Games like XCOM 2 will still retain their original mod launcher for players who enjoy community content.

Go into the directory where Steam keeps your XCOM 2 mods, which should be Steam\steamapps\workshop\content\268500, and then open the subdirectory for any mods you've got installed.

XCOM 2 for Steam is a special case, 2K said, as the launcher included Mod support functionality. Of note, these changes will let you enable mods on the Steam Deck.