I just checked, and yes, you can use Steam Controller to get through UAC prompts. The joystick mimics the arrow keys, the right trackpad moves the mouse cursor, the trigger buttons represent mouse clicks, and A is Enter.
That matters because desktop mode still has customizable keybindings as usual, while lizard mode is completely hard-coded. Lizard mode also (obviously) doesn't support the soft keyboard, which would otherwise have been pretty useful from the lock/login screen.
If you try to stream something that first pops up UAC, the client just sits there waiting for it to be dealt with on the host.
Every time I do that I think there must be a better way.
> @powershell -NoProfile -ExecutionPolicy unrestricted -Command "$sessionid=((quser $env:USERNAME | select -Skip 1) -split '\s+')[2]; tscon $sessionid /dest:console" 2> UnlockErrors.logpp
I've not used it often and mostly given up on Steams streaming stuff. Playing simple stuff though RDP directly usually works well enough for the few times I still did that. Notably pretty much all games launch flawlessly. The drawback is it not being optimized for games, offering a notably worse performance on that front.