One big annoyance for me is that every song I play now shows up in my YouTube history. For me, watching videos on YT and listening to music are activities I do in two very different contexts, mixing them together in a list serves no purpose and makes it a lot more difficult to navigate.

And on mobile GPM had a very simple functionality: a playlist containing all the songs you've downloaded offline. To the best of my knowledge YTM has no such thing, just a playlist _it makes for you_ of offline songs.