I've been using Slacker Premium basically since getting the car. I was getting bored with the selection from Pandora at the time and Slacker advertised a much wider variety.
"Artist stations" are some variation on the artist and some mix ins based on genre. I don't really use those much, but instead prefer to pick an album or make my own station; both are premium features, limited by whatever Slacker has licensed. The Slacker app on iPhone/iPad or browser provides much better customization of playlists and stations than the car app because the car app is based on the Tesla special type of subscription.
The Beatles appear to be a special case. I think Apple mostly owns that license now, because most Beatles stuff disappeared from Slacker. Even searching in the app seems to ignore the concept that the Beatles match an available artist.
For Classical and Big Band, then selections seem a bit limited. On the other hand, things like much of Rock and Alternative/Indie is covered and they seem to have a good Country selection, new and old. Electronica is well covered, including stations for things like Trap but those are a bit limited to 'popular' items within each sub-genre. Oddly tho, I think their House selection is pretty good.
Licensing impacts listening selection too. For instance, there are a couple songs missing from the Run The Jewels2 album if I try to listen to the album and about half the Rush albums, including things like Vapor Trails, are not available to listen to the album itself tho the songs might come up on a 'station' that plays random Rush stuff.
One Premium feature I'd like added to the Tesla app is the "favorites" station -- you can favorite a song but the app doesn't give option to listen to a station of random favorites.