That is far more limited selection than at Spotify, but TBF there is no reason to compare the service given that Amazon Prime Music is bundled into Prime. This service was never going to be a threat to Spotify any more than Kindle Owners Lending Library is a threat to Scribd and Oyster.
It's not clear what terms Amazon negotiated with the major labels, but the NYTimes reported that Amazon is paying the smaller labels and indies from a pool of $5 million which is split based on the number of times each track is played. As you might recall, previous rumors suggested that Amazon was funding the entire service from a $20 million to $30 million pool which was going to be paid out in much the same way as Amazon currently splits KDP Select among indie authors.
While this service may lack new releases, but Amazon is doing their best to overcome its limitations. Rather than focus on tracks, they're building the service on playlists with the goal of keeping you from hunting for a song and not finding it.
That's a good idea on Amazon's part, and once Amazon Prime Music is operational I'm sure it's going to prove a good source of music.
But at the moment the service is unavailable. While the website is up and running, I can't actually get it to stream any songs. I'm not sure whether the service crashed under the load or simply hasn't been turned on yet, but as I sit here writing this post it is nonoperational.
That is an annoyance, but it's not a big deal. I'll just go back to my preferred free streaming music service, Youtube.