I'm constantly having 3 major issues with Spotify and Chromecast:
It constantly loses connection (every 20-30 minutes or so). There are 2 kinds of disconnects that happen:
Spotify loses connection but the chromecast keeps playing. You have to reconnect to change songs etc.
Spotify loses connection and the chromecast quits playing. This happens more with the web player.
Can't start casting with the desktop or android app, I have to start casting using the web player in Chrome, then I can use the desktop/android app (coupled with issue 1 mentioned above this is very annoying)
Can't adjust volume with desktop or android app, you have to use the Google Home app