I have used the postal service in some fairly uncommon ways, and I have never had a package go missing or not meet their SLA for Priority Mail.
You can send a package to pretty much any town in the country for <$20 and expect it to be there in 3 or 4 days. It’s the next best thing to teleportation, unless you wanted to get it on a Sunday.
They've lost plenty of other things besides. I once sent $100 cash in a fun colored envelope containing a birthday card. The card arrived scotch-taped closed with the money gone. I know it wasn't an accident where it got ripped open by a conveyor belt or something because they re-seal torn envelopes in a plastic bag when that happens. That was more an annoying than headache inducing, but UPS has never caused problems for me like those and I use them by an order of magnitude more than USPS.
The worst part isn't that those things happen. Theft and accidents can happen anywhere. The worst part is that the USPS takes no responsibility and could not care any less. So what if a postal employee stole money from a birthday card? Sucks to be me. I can't send letters any other way. The USPS has a legally protected monopoly on letter delivery. So what if the hold time is 9 hours? Who else am I going to go to? The post office is built on bureaucracy and you and I pay the price for it.