Windows Phone has probably the worst music player in the history of music players! Even the alternative ones in the Store are useless. For me seamless music play in the most important. I've bared it with W8.1M but in W10M its horrible. Made the decision to switch to a Xperia X once its released.
I've lost count and how much feedback I've given on this thing.Everybody here who has an issue with Groove, please give MS feedback on the problem. Squeaky wheels don't get grease if they aren't heard.
I don't really "get" Groove Music, but from what I can tell it's mainly, or perhaps exclusively, a cloud-based service. Is that right? I do keep my music in OneDrive, but I prefer to play music stored locally on the SD card. Groove doesn't appear to do that, or if it does I haven't figured out how. So I downloaded Core for local music playback, and it seems to work fine. If someone knows of a better player for SD-only music, I'd be happy to hear about it.