I just touch mine and it felt super hot so I'm wondering if heating is an issue with chromecast? seems like it's boiling, the area I have it in is really cool as well.
I saw some discussion today about users whose Chromecasts might shut down if they get too hot during an extended (like 4+ hours) play session but I haven't experienced that myself.
The Chromecast does get quite warm though, it's got a tough job to do and no active cooling. I wouldn't really worry about it too much unless it does shut down, and then maybe just take a break for a bit or turn down the quality setting in the Stadia app to see if it helps.
Funnily enough, I had a similar concern. After playing for a short while - I happened to touch the Chromecast - and it was very hot indeed - to the point it was uncomfortable to hold in my hand. Not much else to add other than - yeah, it concerns me a little! It does make you wonder how long it will last pumping out these sort of temperatures ...
"Google explains in an email to 9to5Google that there is “no thermal throttling issue with Chromecast Ultra.” Apparently, Google has not observed this shutdown issue due to overheating during “extensive testing” but did not offer a reason why these Chromecast units may have shutdown while playing Stadia."