I've shipped and received packages internationally numerous times over the years -- to Asia, Australia, Canada, Europe, NZ -- and have only ever lost one shipment. One. Back in 1984.
No organization is perfect, but the USPS is IMO dependable.
As for switching services -- not a direct experience -- I've read from time to time that shipping through DHL, FedEx, UPS, and others will generally increase your chances of paying Customs tax. Thus, I stick with USPS -- and shipping onesie and twosies -- I don't ever recall having to pay taxes -- nor did my recipients.
So that's actually a plus for USPS and a potential minus for other couriers/shippers. But I would say these giant shippers are all generally reliable.