Write up from Shimano is that they have made significant improvements over the Dura Ace Di2 system,
The original Dura Ace Di2 four-wire system has been replaced with a simpler two-wire harness for Ultegra. Cable diameters have been reduced by 3mm as well, dropping weight and allowing for a lighter profile.
All cable connectors are now identical, meaning you don't need to mess around with the specifi cables for front and rear. The shape of the hoods has been changed for better ergonomics, an adjustment that should have been introduced on the Dura Ace version.
The junction box has been overhauled, making a more discreet external or internal cable routing.
Both mechanical and electronic versions of Ultegra will receive a cosmetic update in the form of a new glossy gray color