Be glad to explain and it is not so much in the shifting as in the cable angle. The 6800 rear derailleur has the cable going through to the pinch bolt and a greater angle. This made the shifting not quite as smooth, but it would cause more wear of the cable. As we know the rear derailleur cable and loop takes to most beating on the drivetrain shifting. The R8000 has a much straighter cable attaching to the pinch bolt that is much easier on the system. I don't know what others have experienced but this old mechanic like the set up much better. I can go years without replacing front derailleur cable but the rear derailleur cable and loop gets changed at least every 6 months and I ride many miles. I have no complaints on that but just a fact.