Plex has worked mostly seamlessly for me on my iPhone. I listen to synced music all the time at work and in my car, but only from Apple devices (work iPhone 6 and my five year old iPad). It's definitely concerning if you're having issues with it on Android. At home I'm to the point of streaming Plex rather than using iTunes, even though the music is local as well.
Maybe I'll hold off on upgrading from a monthly Plex plan until I know it works for my future phone.