I don't think it's on the tracks that is the problem; it's him LIVE. Have you ever heard him live? After you've done that you can tell his voice on the album is auto-tuned. It's sad since Christina's is pitch perfecto.
I think most of this hate is coming from post-Karmacode fans. His voice is perfect on all of the albums prior to that one. And don't forget that that album was meant to widen the fanbase and get more pop fans, which meant more emphasis on Christina's voice and even then they contrast well.