After 15,800 miles since Fall of 2016 (mostly road, some gravel, 3 winters, some wet riding, and tons of shifting), my LBS had to replace the Ultegra Di2 rear derailleur, which had some internal mech failure with the motor. It went from working perfectly while JRA, to having the catastrophic problem. No gradual degradation. The symptom was when trying to shift to the big cogs, it would instead immediately shift to the smallest cog, and then you would just hear the motor retry unsuccessfully over and over, twice per second for 10 seconds, then quit trying. It would no longer shift to a bigger cog at all.
Other than completely failing at the end, the UDi2 drivetrain was flawless otherwise. And it's getting replaced with the latest R8050-GS derailleur, which hopefully has some refinements and improved reliability.