As a Steam Deck owner I'd say having Linux on the Ally isn't enough and it isn't what makes the Steam Deck great either.
Valve took the time to make features like instant resume and developing a console like interface. The Ally is missing things like that and until has that I'd still consider the Steam Deck a better experience
For mouse heavy games like 4X titles or building games such as Cities Skylines, the lack of trackpads will severely hamper the ROG Ally, IMHO. Touch screen is not a good substitute.