Yep. Your second video (loving me 4 me) is a perfect example. She sounds great there.
It's something I don't have with other great singers, like Beyonce for example, or Lady Gaga, or Leona Lewis for that matter. When they go into the loud and high notes, they still sound great to me. Their voices remain warm and full, Beyonce's especially. Christina's otoh gets shrill -even if she's still technically perfect I'm sure-. Bit of a shame.
Fully understandable. But the problem for me is, that I'm not that big a fan of soul music. I like pop music. So Christina sounds better when she sings soulful, but I like her music better when it's more straight-up pop.
Fuck Your Body also suffers a bit from the "loudness" of her voice. However, I do like how her voice cracks a little during the chorus.