This problem is not exactly with Stadia, but mostly with Ubuntu 18.04 and Google-Chrome.
I have a Switch Pro controller connected with my computer with Bluetooth, and my Firefox recognize it fine (tested at https://www.html5gamepad.com) but google-chrome don't.
Since Stadia (unfortunately) only works on Chrome I was unable to play using my gamepad (it doesn't recognize any gamepad connected).
Anyone has any idea on what should be the problem?
@ThOR27 Yep, sorry, I misread your initial post thinking you were talking about the Stadia controller rather than the Switch controller (which you did clearly state - I just haven't had enough coffee today).
What version of Chrome are you using, and how did you install it? You might want to reinstall from here just be sure:
Good news !
I just use for the first time Stadia on Linux GNU/Debian Bullseye and I was in the same case.
Some details :
First, I tried Chromium but it crashed with the game power rangers, it stays in loading after the some screens...
Secondly, I installed Chrome (Official with repo id to be up 2 date : current version : Version 85.0.4183.83 (Build officiel) (64 bits)), better, I saw the screens and the game loaded etc, I tried and re-tried to connect the controller...
Impossible, I don't know why because it comes with Wifi, ok with the chromecast ultra but not on chrome on Linux...
I read your posts and I read usb. I tried USB and I can say, today the 2020, 3th of September it works Guys !
Thank you !
See you soon on Stadia !