I'm surprised to see this thread resurrected from the dead. Updating from my March, 2018 post above . . .
I now have over 70,000 miles on Di2 groupsets and I still have yet to have any Di2 related problems.
BUT . . . the guy I sold one of my old bikes to had a front derailleur break. It had nothing to do with the motor. On a previous generation Ultegra Di2 there was a cast metal link that attached the actuator to the cage. That tiny casting broke. He still rode out the century without any problem -- he was just limited to one chainring. Rather than try to obtain the part, he decided to upgrade to the current Ultegra Di2 front derailleur. The current version has a flat metal "angle iron" style design for that linkage. How long did the cast link last before failure? 3 years and about 20,000 miles.