It is supported but the chromecast app is not the best implementation I guess. I use chromecast for multiroom audio myself and it works pretty reliable for me. But I also assigned all chromecast devices a fixed IP address. Made sure they have good wlan coverage and I automatically restart the chromecast app every morning by flow.
Up until now everything keeps working.
If you cast to a speaker group you will have another problem to take care of. Chromecast automatically elects a device to be master for the group and that is based on uptime of the chromecast device sorts of. So If you have a group added as device, try to make sure that device is always on. The chromecast app seems to have a problem when the master changes, effectively rendering the speakergroup device unavailable.
Maybe this helps, maybe not. fingers crossed
