Well, I revisited MusicCast and must say I like it very much. I really like the Spotify integration, which is just Spotify Connect that works. All my earlier negative comments about library management in the MusicCast app still stand, though.
If I want to listen to my own, quite extensive library, I still use my old trustworthy Squeezebox ecosystem, hooked up to my Yamaha CX-A5100 or Yamaha XDA-QS5400. However linking to other rooms is done by MusicCast now, and it's rock solid, without delays. This is one area where MusicCast performs better.
So I now use my Squeezebox as a high definition audio component in my Yamaha MusicCast setup. Good library management using the Squeezebox, good audio quality / syncing using MusicCast. I'm able to link/sync my TV audio to other rooms now. (playing music videos from You Tube etc.)
I suspect that's what Yamaha wants us to do. Let the geeks (audiophiles), using their own local music library instead of streaming services, use their system of choice for library management (Sonos, Squeezebox, Roon, etc.).
I even added a Yamaha MusicCast Vinyl 500 to my setup now. Playing (and syncing) records in every room.