You people are literally incapable of staying objective, aren't you..
Understand that evaluating and judging the ability of a person's musical skill (vocalist, pianist, trumpeter, etc) to determine superiority in a professional field relies LESS on your sentiment preferences and MORE on actual quantitative traits that can be "measured" by concrete evidence..
So basically when you want to label who is the BETTER singer, you have to have actual evidence that would PROVE and SOLIDIFY your point. It's like writing a paper: you have to state your thesis, explain it, have proof (statist. quotes. character traits) to back that up to anchor your claim.
Same thing goes for any instrumentalist. Remember the voice is an actual instrument to a vocalist just like how the piano is an actual instrument to the pianist. If I'm a musical director and I have two lead roles for a female vocalist and pianist and I wanted to find the best one, In a professional sense, For a vocalist, my job would be to find the singer who are the master of their craft, which depends on the actual objective qualities..NOT who's timbre makes my heart swoon.
In saying that, I say this..It's perfectly ok to say that Beyonce is technically better than Christina as a vocalist, though you may prefer the X as a singer/artist because you find her a bit more emotive.
Range is not whatever pitch you can muster from your bones, but the ones that can be sung with comfort, connected to the body with precision and utilized efficiently with control..
and to answer the OP's question, She's OBVIOUSLY an expert in vocal pedagogy and gets UNDERRATED because not everyone is studied to it to recognize her actual skill/ability
I used to prefer Xtina's voice, specially during Stripped/B2B eras, but her weak technique damaged her voice way too much and Beyoncé is great at preserving her vocals and has a better control so she wins.
They both oversing way too much tho.
Christina literally can't sing. There's a difference between having a good voice and being a good singer. Beyonce may be one of the most boring and soulless vocalists of all time but at least my throat doesn't hurt when I'm watching one of her live performances