There are three that are insanely bad.
One seems fixed. Not sure. When you go from wifi to mobile, even if playing offline songs, it stops playing. So bad in mixed connectivity.
Two is insanely bad. Add a song to Up Next and it gets added a million times. WTF.
Third the Watch doesn’t keep the same AirPlay settings as the phone. If you are airplaying from the phone then make a tweak from the Watch, the song starts playing out of the phone speaker.