I honestly don't know. But every developer I've seen talking about publishing on Stadia, also talked about working with the team, so there were definite considerations that they needed to take into account.
There's stories like this [0] which suggests that the performance for Stadia was different than other platforms you might deploy for. The threading behaviour is a little bit different than just a VM, which is to be expected, but can come out surprising. So there's probably Stadia-specific patching for different games.
I can “launch” (pun intended) AAA title on a cloud VM in the time it takes for steam to download the game.
I haven’t used stadia; honestly curious.