Nikkei says the mockups have a low building quality. The home button doesn't fit properly leaving a gap and the display isn't transitioning seamlessly into the aluminum shell. The site says that the iPhone 6 may have a curved display that fits perfectly creating a seamless transition from the aluminum to the display.
Most notably, Nikkei (via GforGames) says that the antenna breaks found in all the mockups are not part of the final design.
Last but not least, the source has also mentioned that the white stripes found on the back panel of the mockups are not going to be part of the final design. For now they might only mark out the area where the back panel will “break” into multiple pieces, similar to the iPhone 5S.
Apple is expected to unveil the iPhone 6 this fall in 4.7-inch and 5.5-inch models. Please follow iClarified on Twitter, Facebook, Google+, or RSS for updates.