While music can be categorised (though what use that is, Lord only knows), artists cannot. Metallica in fact have 2 or 3 excellent ballads, that would not shame any folk collection (Look for the acoustic version of "Nothing else matters").

Artists don't move within one genre or category. Why should they? Music has no bounds, only people's minds do. I have written (just among my published stuff) in the following "styles":

English Traditional
Mediaeval
Blues
Rock
Chanson
Church Hymns
Greek Traditional
and there are a few that would not fit in any of the above either.

So what am I? On which shelf do you put my albums?

Songs may be categorised. Albums and artists, not.