The only thing that seemed to be their fault was the reservation cancellation because of routine maintenance...but then again routine maintenance does not mean scheduled maintenance. So it is possible that the proximity card reader stopped working or there was a flat tire or some other urgent matter.
Maintaining and renting cars remotely is a crazy difficult service to provide and yet they pulled it off. I would hardly call these flaws Zipcar's "darkside" although they could definitely improve in some areas.
Competion would improves this space in two ways. The standard more innovation, better features, blah blah blah way. But for this type of rental car service the more companies there are the better selection for the consumer because you are more likely to have a car in close proximity. It isn't feasible to put a Zipcar everywhere, but if more companies start trying to compete then at least some of them will put cars where Zipcar is not, which means you don't have to walk 1+ miles if you live in downtown Palo Alto :)