I use 4 Google Home speakers in different rooms to listen to Radio 4. I have the speakers linked via the app in a group called Home.
I was used to saying "OK Google, play BBC Radio 4 on Home group" and it'd play out of all speakers and they'd all be in sync. Great, and cheaper than a set of Sonos speakers.
Over the last few months it's been getting very frustrating where I'd say the line above and then - nothing. About 70% of the time it wasn't working.
I rebooted all the speakers in the hope of them updating their software and that just resulted in a more or less 100% failure to work.
Well I've just discovered that if I say "OK Google CAST BBC Radio 4 on home" it works as it should so my frustration has been eased for now and I hope someone else may find this tip of use.