It varies widely based on the sorting center near you, your local post office, and your specific mail carrier. A friend of mine used to work in a sorting center and said it was the most brutal job he'd ever had because it involves a lot of physical labor. He said he never saw anyone purposefully mistreating mail, though he did see one or two people busted for stealing packages. He also saw several accidents where automated machines would mangle a package, once while he was working the station. IIRC he had to carefully pickup all the pieces he could get out of the machine and pack it in a new box.

Everywhere I've ever lived all of them were great and I've never had a lost package or any problems.

The only negative experience I've had is the substitute carriers here in SF; when my regular guy is out the sub loves to claim he tried to deliver the package without bothering. When someone is home all day it is a bit difficult to believe they attempted delivery without leaving even a note.