Valve reports that the Steam Machine will support inputs from up to 4 Steam Controllers [1], so presumably they are updating SteamInput to handle that.

[1] https://store.steampowered.com/sale/steammachine

I'm not sure what you mean ?

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 ?