I think the ROG Ally is DOA. Streamers and ROG addicts will run out and buy it immediately. Then it'll just collect dust on shelves. I bet it launches at $700 starting and gets discounted a month after release.
While the pricing, performance, and size of the Steam Deck are important features, there's one thing that Asus cannot dream to compete on -- software support. I'd rather have a Steam Deck and know there are dedicated, internal developers at Valve contributing to the OS and drivers than buy something from Asus with nebulous development support.
I do think Asus did a good job with this gaming handheld. Good design, good specs, and likely good performance. That's not enough for the high price they're going to demand.