My experience of Google Play Music is that it works well when it works. But something it really likes doing is not playing. Our running joke is that every time a butterfly flaps its wings anywhere in the world, Play stops playing. While hyperbole it does happen a lot, especially if there is less than perfect networking (eg while driving). But it will also stop playing with perfect networking, usually at the end of a song for no good reason.

Spotify is far better at actually playing under the same conditions. If anything it is too eager (there was that time I had to tell it to stop 4 times). However it is a huge pain if you want it to just play music without having to babysit what it plays.