How reliable are QS Top Universities rankings?
I was told that universities in USA are generally better that the ones in Italy. 


This statement, in its given generality, is false. The US, like most places, has very good schools, ok schools, and a reasonable number of pretty bad schools. If you explicitly are looking for very cheap schools, I would not assume that the remaining selection is necessarily any good, just because it is a "school in the states".

POLIMI is a pretty good school in CS. To be honest, if I had the choice of going either to POLIMI or to a low-ranked US school, I would almost certainly choose POLIMI.