And, if i'm reading their site correctly, requires you to have a gaming PC with good networking. It's not even in the same ballpark as Stadia!
What you should be comparing Stadia to is GeForce Now, Amazon Luna, Shadow, which provide the computing power as a service. Stadia eschews Windows to run the games, making it cheaper ( their free tier gives you 1080p/60fps for free, you just have to buy the games on the Stadia Store) but limits what games they can run ( they have to be made for Vulkan). The downside is that you have to buy the games for it, but it supports ubisoft+ ( with an addon on the Ubisoft side). But you don't complain when you have to buy games anew when you buy a new platform ( be it a PS, Xbox or PC), and the same logic applies here.
GeForce Now and Amazon Luna are still a Windows server box behind the scenes, with the associated slowness and a bit clunky integrations ( e.g. GeForce Now requires you to re-login to Uplay or Epic or Steam every so often, which is painful with a random password and MFA, and no copy-paste available).
Shadow provide you full access to a Windows box, with the associated issues ( game updates and downloads are up to you, disk space is limited, etc.).
Stadia has some downsides, but IMHO fares very decently against the competition. I have both it and GeForce Now, for different games, and IMHO Stadia's UX is slightly better.