Downsides are no native client (same webview approach as Spotify), not as strong recommendations (though slowly improving), good but slightly quirkier UI (also slowly improving). For me the trade offs are worthwhile.
[0] eg. https://resources.onestowatch.com/ethical-music-streaming-pl...
Fuck Spotify and their rotten leadership. Was amongst the early adopters (2009). I'm sure they won't care for the loss, and I won't miss th when they're gone.
Apple Music pays artists better, too.