I am finally biting the bullet and going through my library. I never bothered before, but the My New Music Mix was way off base for me. The favorites one was spot on though.
Edited to add that I'm not sure if play count is a factor in any of this, but honestly I wish that's what it all was based off of. My tastes are varied, and I never really know where I'm going to be headed, but it would be nice if it would just take what I've been listening to without input on my part and just show me things, a la Discover Weekly.
Since I primarily listen to music on Apple Music, my Discover Weekly lists on Spotify have gone to the dogs, and Apple Music seems to need a lot of pushing from me to get it right, which I'm just generally not taking the time to do.