I just joined as a Beta user as well. I have not tried other users playlists. However I did have a question about custom playlists. It seems to limit you to 1,000 songs. Also, when I try to play the 1,000 playlist through Apple CarPlay it gets limited further to 500 songs.
I sent a note to support and they said they are aware of the issues and are working on it.
Has anyone else experienced this ? How about non-U.S. subscribers ? Is this an issue outside of the BETA program as well ?