Claudia Friedlander is a classical musician and voice teacher. She was asked her opinion of five different male heavy metal singers. (The link also plays samples of the music that she was analyzing.) Notice that classical aesthetics contains principles that apply to every kind of music, without necessarily demolishing the more popular genres:
On Bruce Dickinson of Iron Maiden:
I have nothing but admiration for this singer. Listen how he starts off with a soft growl, then moves seamlessly into a well-supported, sustained high full-voice sound that then evolves into an effortless long scream! His diction is easily intelligible, regardless of the range he’s singing in or the effect he’s going for. He achieves an intensely rhythmic delivery of the lyrics without losing legato and musical momentum, something a lot of classical singers struggle with, especially when interpreting the many staccato and accent markings that crowd scores by Bellini, Donizetti, etc.
On Ozzy Osbourne:
This is a singer with decent diction and good musical instincts but no command of vocal technique. He is massively over-adducting his vocal folds while driving enough air through them to get them to speak, but his throat is so tight that there is no flow or resonance. His rhythmic punctuation of the lyrics is very distracting, in contrast with Singer #1 [Dickinson] who delivered his text with rhythmic accents that served, rather than detracted from the flow of music and poetry. It hurt my throat so much to listen to him that I was tempted to ask Cosmo how long his career lasted before he either washed out or needed surgery. The entire range of his singing is contained within a single octave – with the exception of the moment when he yells “Oh Lord!” a little higher, in my opinion the only quasi-free vocal sound on the entire track.