Some People can’t add them to existing Sonos system set up new systems. Basically they can’t get them to work. 
 

To extend that, it then must be the case that many people buying Sonos kit today will not be able to get it to deliver music? Even via the Spotify native app that can cast to Sonos kit?

Yes. They can’t update the firmware and support either can’t help them or can’t get through. Also seems to be affecting other units but updates yesterday may have fixed them.