Thank you everyone for your replies, lots of interesting stuff has been posted in this thread.
If I had only the classic shifter on each brake lever, like you have on a mechanical groupset, then I think I wouldn't even be thinking of going electric. But as I am spending more and more time on my aero bars, the idea of being able to add satellite shifters is very appealing. Adding an extra pair (or pair of pair) of shifters, does remove the issue with "what if the my primary shifters fail". You can't have multiple indexed shifters for the same set of gears with mechanical cables.
So based on the answers people have given in this thread it seems primary failure modes can consist of (but are not limited to)
- Dead battery - Keep it charged, problem solved. Long trip, carry charger
- Broken cable - It appears that the general consensus is that the cables are not as durable as they could be, carrying spares, and having the wherewithall to perhaps replum the wiring at the road side can mitigate this slightly. I'm looking at Di2 for an S&S coupled bike, which makes the internal routing thing slightly more interesting, but also more accessible.
- Crash protection mode - Knowing it exists and how to get out of it, solved.
- Total device failure - Well this can happen to a mechanical unit, tho admittedly the electronic one is more complex and thus does have an opportunity fail in ways you may not be used to.
Now in all this there comes the question "What do the pros use?" Well that's an interesting one. Cos there is an assumption that I'm a roadie... The riders who inspire me (tho I won't go so far as to say I want to copy them, be them, or emulate them, but they do inspire me), are not riders who ride primarily in events run under UCI rules... So it doesn't really interest me what nibali or contador used. What I do know is that James used Di2 to win the TCR last year. Sarah seemed to be using mechanical on her IPWR attempt. Kristoff also seems to be mechanical, as does Emily. With the exception of Emily, I can't find any detail on the reasoning of each of those riders for choosing one over the other. Given that AFAIK, most of those named are buying, or at least actively choosing their bike components, I wonder how much cost is a factor. If they had a choice between di2 and mechanical, for the same price, which would they choose?
Anyway, thank you everyone for your input. It's given me much to think about. I've got a couple of months before the frame is finished and I need to decide for certain where to put my money.
Thanks
J