lthough everything may happened, in my opinion it's not frequent.
Have a look at some of the threads here, Looker, and talk to people from abroad who live in Poland. Sadly theft of incoming mail is so commonplace that some countries' postal services now have contracts with couriers rather than risk Poczta Polska.
I've been lucky - I've only had bank cards stolen which are easy to cancel. Others have some real horror stories.