Every "good" vocalist can decently sing a ballad; it takes actual skill to effectively deliver an uptempo
Jennifer cannot be considered a "top" vocalist when she's incapable of singing uptempos..
https://www.youtube.com/watch?v=ajW8mEzsV8I
(1:30-2:15) - Here is where your see the obvious discomfort..lazy timing and instead of sounding full and warm in the 3rd/4th octave, EVERYTHING comes out dry, foggy and without any support. Then on the other end of the spectrum she's squeezing thin, shouty belts out of her forehead..Without real control of the breath and an uneven mix she always sounds heady and never projects her sound forward; it merely sits on top of her face sounding "loud" instead of having true resonance). This is why when she tries to cut through Jessie at 3:31 she starts going slightly sharp because she's forcing so much pressure to get that note out because it's PLACED soo high in her larynx..hence the distorted sound.
She phrases everything like she's singing a dang ballad; her poor sense of rhythm), which means she has virtually no versatility, but Beyonce is the one considered to have "limited" ability
and dont get me started on her 6th octave notes with such ease 
There were several flat E5s in that video by the way lol And I DARE you to tell me 2:40 wasn't lipped. Come on now. A Mezzo. At least Christina is bold enough to take the fail, she doesn't circumvent it with gimmicks 