@Matt1 For now, at least, the controller's microphone is never used for voice chat:
https://support.google.com/stadia/answer/9609774
I don't know if that's something that might change in the future but I don't really expect it to. The controller's mic is probably good enough for Assistant to hear you but may not have the best audio quality for voice chats.
Feel free to use the in-app feedback option to let the developers know what you think about it though!