I've been using JRiver for Android for a couple days now, and it's actually working pretty well. My only thoughts on things missing are...
1) Getting gapless playback working. This is the big one for me as I listen to albums (e.g. Pink Floyd) that segue into each other. Right now there's a gap between songs.
2) The inability to remove items from the library + media files used for other apps, e.g. Hangouts (Hangouts Call and Hangouts Message), are being added to the library (under Unassigned) when they shouldn't be, IMO. Can the Hangouts files be added to an exclusion list so they're not imported?
3) The persistent notification stays even if you close/swipe away JRiver for Android. IMO, the notification should only appear when a) JRiver for Android is in focus and/or b) JRiver for Android is playing something. The only way to fully close JRiver for Android is to force stop the app.