Earlier quoted context omitted.
Isn't the probability of something going wrong higher if the delivery infrastructure isn't staffed with full time people? I would guess that mishandling, loss, misdelivery, theft would all be more likely under the flex model.
But it would also be a lot easier to track what happened to that package and pinpoint a specific individual for the loss/theft/misdelivery, since Amazon has the driver's personal info, GPS location, and list of packages that they are delivering.
With flex, it's likely the same quality process, only the workforce is larger and more likely to churn. So no matter what the performance of the quality process it has some non-zero time to detect and correct worker performance, so I don't see how one avoids a larger rate of delivery faults just from the workforce dynamics.