I switched from Spotify from YT Music solely for UX reasons. Spotify is a weird flimsy thing to me (or at least was, back when I rage-quit it). Things like, their Android app didn't even have a "play album" button. Random simple stuff just was made needlessly hard. Queueing was weird, it seemed to nudge you to shuffling / algorithmic playback, they had this weird podcast thing going on that was just in the way, and so on.

YT Music on the other hand, has excellent UX in my opinion. This surprised me, given Google's generally mediocre UX design, but they really got a bunch of competent people on this one. All the basics work the way you'd expect (and that's not trivial to get right). Play, queue, play next. Play album, shuffle, it all just generally does what I expect it to do and I can mostly find the buttons I want easily. You can turn off autoplay. Gapless album playback is on by default. It.. just works!

Also I find the algorithmic autoplay to be pretty great, found some great new artists that way.

The fact that the catalog is bigger because it includes weird bootleg recordings and live sets and anything music-y ever uploaded to YouTube, is a nice bonus. But for me, the UX sells it.