I used Play Music and Inbox as well, and also just switched (3-ish months ago?) to an iPhone.
The YouTube Music app is not that bad, but definitely missing a feature or three that I used a lot on Play. It also seems to need to be manually started every time my phone connects to Bluetooth whereas Play would start automatically if it was the last thing that was playing on that BT connection.
Youtube music's biggest drawback is being required to subscribe to listen with the screen off. I was willing to change interfaces since I thought google play music was reasonable. Then my free trial ended.
I use Spotify for 95% of my listening. Their ability to sync songs that aren't on Spotify though is pretty bad, and that's why I used Play Music. There are just some albums/bands that I want to listen to that aren't easily available without having the raw MP3 somewhere.
The YouTube Music app is not that bad, but definitely missing a feature or three that I used a lot on Play. It also seems to need to be manually started every time my phone connects to Bluetooth whereas Play would start automatically if it was the last thing that was playing on that BT connection.