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.