People don't complain about USPS because they suck at delivering to my "island" home in the middle of a swamp. They complain about USPS because somehow they seem to get a lot of employees who just don't try.
In my experience, Fedex and UPS deliver when they say they will. USPS "attempts" deliver all the time only for the package to fail to arrive. I live in the urban part of a small city/big town. There is essentially never a reason a package can't be delivered.
My latest gripe concerns the USPS person that delivers my daily mail. She loves to catch me outside and let me know that she doesn't "legally" have to deliver my mail if there is snow on my steps. I live in New England. Everyone has fucking snow on their steps.