The game library was the big issue for me, most playtime was quite smooth for me. But to a large extent it's publishers' faults, Geforce Now has the same problem with games they had before getting removed because publishers/devs opted-out.

I think Stadia had hoped that it could become big enough quickly, with the backing of Google's name and capital, that it could negotiate some good deals and build a big library quickly, to the point that there's so many gamers on Stadia that you can't not publish there anymore without losing a lot. Netflix and Spotify did it, too.