Over the last 15 or so years I have shipped thousands and thousands of packages by USPS and they have lost exactly ONE. I've shipped to almost anywhere you can imagine including rural China, Russia, APO, FPO, etc.
I also only get one or two a year that are significantly delayed. This year items are arriving on average before their estimate date.
If you are having problems with USPS you have to look at what you are doing: How are the labels being produced, what type of boxes, envelopes etc you are using and how you are getting items into the mail stream.
If you're going to use USPS you have to do things their way. This means you read all the facs and find out everything about restrictions and recommendations.
If your items are not machinable it could mean a delay
If your items don't have standardized addresses it could mean a delay
If you don't have barcodes it could mean a delay
If you have something that is worth more than $10 do not use 1st class
My favorite USPS story.
One of my customers in Chicago had a very urgent order. I shipped it at 9:30 am by express mail with guaranteed delivery by 3 pm the next day. I call the customer the next day at 4 pm to make sure he received it. He said "No". I started to get upset and he interrupted me to say "It got here YESTERDAY." They somehow managed to get the package their the same day it was shipped