I base it on my observation of them and their employees over about 10 years. I see how they handle hundreds of my packages per week. I've dealt with dozens of USPS workers from carriers to post masters. And I have seen many facilities up close. I don't think the people are any worse than at UPS or Fed Ex, but there are too many of them and the union should not exist.
Viewing "facilities" from across the counter and then implying you have spent many hours watching USPS operations inside of dozens of main facilities is not exactly a qualification. Seeing how they move your packages from the counter to the hamper, or from your dock to a truck doesn't qualify either, but nice try.
I am no fan of unions at all. But I have seen good employees saved from gross injustice by them. I have seen slugs defended by them also, unfortunately. I think it is fair to say that the problems caused by unions and poor employees are no different across USPS, UPS, FedEX and others, so why bring it up and pick one organization to malign?