Hmm. I assume that you haven't read threads. Do you refer to bricked devices and problems with free space for upgrade. I would roughly say that in 80-90% of cases reason is that user has downloaded/installed something form extras testing/development without realizing that after pressing button he/she is on way to reflash in near future.

There is worrying amount of people who have used NSU and their device is bricked. But who really knows what is the problem there.