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.