[somat]

Based on personal experience my guess is that the unreliability would be in the battery not the drive train.

Or more precisely put, batteries are a sort of black box they ether work or they don't work but either way you are not going to be able to open one up and find out why. that is, they are a high cost unrepairable item on the vehicle and this is a huge liability.

[BirdieNZ]

Batteries aren't unrepairable; you wouldn't open one up in the middle of the road to try fix it but at the bus depot with enough volume of battery electric vehicles, they'll have reason to hire repair technicians that can refurbish and repair batteries.

[AtlasBarfed]

Obviously anything has a bunch of single points of failure, or catastrophic means of failure, but a battery isn't like "one engine". It's basically hundreds of little power modules wired in parallel, so an individual battery cell loss shouldn't bring down the whole pack.

So a battery pack should actually be heavily redundant ... assuming the pack has enough modules for a loss of vehicle to get to some charging station.