When Google Play started a "radio station" based on a song, the songs that came after were changed on each iteration based on your likes/dislikes. Maybe they just shuffled the same list each time, but it kept it feeling fresh. When you ask YouTube Music to play a song, the same set of songs will always come after. Not very good for similar song discovery.
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.
They moved my favorite songs to one of my THREE youtube profiles.
They couldn't move it to my main because it was legacy, but that was where I had youtube premium/red whatever. So then I needed to switch between accounts. Not fun while you are driving.
Oh then they stopped premium so I started hearing ads. Not what I want when I'm trying to calm my screaming kid in the car. Oh, and I need to keep my phone screen on.
Many of the songs aren't the same as on Google play.
Google seems to be moving toward search/email/YouTube. With few other products. Probably good, they can't handle it.