The mood and style 'radio' features depend on proper tagging. It does have an extensive DB to match against, assuming your artist/album/song tags are correct, but depending on the popularity of the artist, you may find gaps where some albums aren't included in a genre/mood shuffle when you think they should be.
The metadata can be edited by hand, and you can even invent your own 'moods', but the current Plex UI to do all this can be slow and tedious.
Still, when tagged well, the radio feature works well. I had a "Dramatic Radio" playing off my Disney folder and the song selections (including some background music items) was impressively 'on'.