Yes, certain aspects of singing are subjective, mainly beauty of tone and depth of emotion...how someone makes YOU feel with their singing, those things are up to personal opinion. However, many other things are quite objective as they can be quantified. Pitch is the main thing...one is in tune or one isn't, no in-betweens. Then we have rhythm, resonance, breathe support, register coordination, phrasing, vocal versatility, etc. All things that are quite technical and because of their technical nature they can be quantified.

As I've said, singing is not just an art but also a science, and like all sciences, it has a set of rules that can be more or less bent, but not broken, when one breaks the rules of singing it turns into NOISE instead of music.

Patti Labelle does have a very acidic, very metallic voice which is an acquired taste, but as a vocalist she is really great. If one can get past the innate sound of her voice, one can appreciate her musicianship and vocal ability, which goes far beyond the natural steely edge of her voice (dramatic soprano).

That's why I don't judge a singer's vocal ability by how pretty their timbre is, but how well they sing. Yes, Leona has a very beautiful voice and in some things she can sing quite wonderfully (her cover of "Lovin' you" by Minnie Riperton shows her voice quite beautifully) but as a technical vocalist she has certain flaws that make her subpar when compared to someone like Debelah Morgan, for example, who has a similar voice type.