Pack items appropriately, and you'll have a good experience with most shippers almost every time.
Throw parts in a box, not packed well, and you'll get lucky a good percentage of the time, but shouldn't be surprised the times you don't get lucky.
Yes, things get damaged from time to time, no matter how well they are packed. That falls into the sh*t happens category. Packages get stolen. People make mistakes. Trucks and planes crash or burn. Insure packages of significant value or where the item is difficult to replace.
I can count the major USPS problems I've had in more than 10 years of selling on eBay and other places on one hand.
Plenty of small inconveniences along the way. Plenty of e-mails from customers asking 'where's my order', when the tracking # could've told them without asking me.
I had a problem last week, contacted the postmaster in the destination city, and the postmaster straightened it right out.
Yeah, packages have gone the wrong direction a few times. I have had packages marked no such address, and when I re-shipped to the same address, they got there the 2nd time, no problem.
I've had packages that I shipped from Indiana to Ohio get scanned in Oklahoma, and take 3 weeks to arrive when it should've been 3 days.
But, I'd postulate 99.44% of the transactions went well, as planned, and everybody was happy.
If you find a particular carrier doesn't provide good service in your area, ask people shipping you to use another shipper. That's no different from not going back to the barber who does a crappy job cutting your hair.