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One thing I'm not sure about with Di2 is how bodgeable it is if it does go wrong. At least with cable mechs if you snap a cable or break a shifter you can trap the cable under a bottle cage bolt or use the limit screws to pick a vaguely useful gear: on the Ditchling Devil a couple of years ago I rode with a guy whose Di2 rear mech packed in (I think it still shifted at the front, so it wasn't battery), and neither of us could work out a way to give him anything other than the smallest sprocket.
That said, Bikey Mikey's not had any real trouble AFAIK.
More and more Di2 appearing on Audax rides.
A huge number of French PBP riders will be on the latest and greatest carbon bling and I suspect there will be a lot of Di2 (you can spot the French riders easily, no luggage as many of them have a friend/family member at each control in a camper van).
There were a few issues on LEL with Di2, but there are those like Mike Lnae (points champion and record holder) who have used Di2 for many thousands of km.
Battery life is good enough for a 1000km + event I believe, but I do know that a couple of riders left a spare battery in a bag drop on LEL, and a couple of riders ran out of juice and had no way of charging.
As @Greenbank says, there are mechanics at every control on PBP - doing roaring trade in new wheels, saddles etc. Difficult to say what expertise in Di2 will be like, much like the UK, some mechanics will be 'WTF is this?' and others will be totally happy with it.