https://thenextweb.com/gaming/2019/11/21/stadia-chromecast-ultra-overheat-issues/

It appears Stadia rocks a little too hard for some of the hardware its purported to work on, in this case some Chromecast Ultras. That’s not likely to be a problem if Stadia becomes as ubiquitous as Google’s planning, but it’s unfortunate considering the Ultra is part of Google’s inaugural Stadia package.

Some users on the Stadia Reddit have reported their Chromecast Ultras overheating and shutting off while they’re playing games with Stadia. The devices run extremely hot when this happens. We don’t have enough data on their reports to figure out if it’s Stadia that’s the problem or a defect with the Chromecasts.