why is USPS MO an "unsafe payment option"?

[WatchGuy1337]

Hi everyone, forgive the naive question:

In the thread "Possible scam on TRF?" (http://www.rolexforums.com/showthread.php?t=120380) people are calling Western Union and USPS MO "unsafe methods of payment". What makes them unsafe compared to a wire transfer? As I understand it, with all three options, once you send the money it's gone, with no way to get it back if something goes wrong (say, in comparison with the protections offered by a credit card, or even PayPal -- and yes, I know PayPal has its own set of issues).

Can someone please explain to me? Thanks!