I am not a Pole, but live in Poland. Umowa o dzielo is quite pervasive but this is not a problem specific to Poland. Portugal, my home country has the same problems just the thing is a different name. The translation would be contractor. As a contractor you obviously have no rights, even so i think this is not the equivalent of US employment.

In both countries the governments are trying to make it quite hard to have people with Umowa o dzielo, which are de-facto employees, but do not pay the appropriate taxes nor are corrrectly protected. The employers also have some reason to do this, the protections offered to the employees often make it a very big risk to hire somebody. Regardless, i believe the southern European countries got it much worse on that front. (Note, the Polish social security model was heavily influenced by the spanish one).