Hah! Missed that one. Steam OS is the Steam Deck’s secret sauce. Even when you have to go into Desktop Mode, it’s okay without a KB+M. I wouldn’t want to use it as a laptop replacement without KB+M and maybe an external monitor mind you, but it’s usable.
I haven’t test driven an ROG Ally, but the though of Windows on a handheld fills me with dread for any interaction that devolves to the desktop.