This question asks "what's the hardest thing to learn in Italian for foreigners", but it is phrased as a request for opinions, and so it has (correctly, in my view) been closed.
However, that question can be answered more scientifically: foreign learners take Italian tests and quizzes all the time, and it should be possible to use some statistics to tell (for instance) if questions on the usage of subjunctive have a higher error rate than questions on the usage of pronouns.
Are there any statistical analyses of this kind? What is, truly, the hardest thing to learn in Italian?