IME, di2 failures are pretty damn rare, but i agree that if you, for example, manage to slice a cable it's not as easy to repair as a broken mechanical cable.

i was riding in a super rural part of the french pyrenees in the fall, and a friend experienced a weird di2 failure--something i'd never seen before. the closest bike shop actually diagnosed the problem and traced it to a faulty junction box, and they popped in a spare. (not the front junction that controls adjustments, for those who know di2 -- the internal junction that just bridges 4 wires.)

anyway, if shops in remote parts of france have di2 parts they're getting pretty darn common. if i were going somewhere that had no support options at all, i'd probably bring a spare wire or a few small parts, but i think it's overkill.

i've seen people fail to fully connect some wires (hard to do), and some cases where the installer failed to follow instructions and left no slack at the shifters (so when the bar shifted after a hard hit it broke a connection)...and one guy who ignored the many warning signs and managed to run down a charge.... but aside from installation issues and a mr. magoo issue, it's a pretty bomber setup.

i've ridden di2 through lots of conditions (mud, snow, downpours) that would cause mechanical shifting to deteriorate; they simply have no effect on di2.