I wonder if Steam will finally implement multi-user sign on for local multiplayer games (like all true consoles). It's something that doesn't get headlines, but a real barrier for enjoyment for a console-like PC. Hate being stuck with 'guest 1' and 'guest 2' or whatever. Many games want each player to progress and without true multi sign on, it just doesn't work. Hence games dropping local multiplayer on PC.
Steam would need to reliably pass multiple controller inputs to the game before your qualm gets addressed
We have been using 3-4 controllers on the Steam Deck lately : Steam Controller, Xbox One gamepad, Switch 1 Joy Pads (together or separate).
There are some quirks with the first time game setup sometimes, but we've never noticed any issues after that ?