I'm not convinced that the ROG Ally is designed as well as the Steam Deck, and I'm less than impressed with Asus in recent years, but I do love open systems. This seems like a good move for Steam OS.
It’s not unexpected at all. Throwing in some drivers and whatnot is trivial compared to what Valve has already done with their OS. This is has always been part of their plan.
Is your question about the task for the end user? If all works well, it might take as long as installing a new operating system on any other device. Maybe half an hour give or take.
SteamOS isnt really open given its got a whole bunch of unreleased patched open source software (probably in violation of Foss licenses) but always good to have more linux friendly handhelds.
I think this was their plan all along. Maybe it wasn't, in any case I was sure it wouldn't be long until we see SteamOS (or something like Bazzite, which is already here I don't know how functional it is) on these things. Why you would advertise Windows on a handheld PC I didn't get at all. The certain titles you are only able to play on Windows (bc poor support or anti-cheat etc etc) usually aren't games you would play on a mobile device anyway.