I cannot answer that question, percentage wise. But I have a friend who spent part of her modeling career in Milan, and she said the organized crime there is far more powerful in their organization and weaponry than the police are and have no fear of them at all. They will run the police right off the road.
Fortunately, the organized crime (she said, maybe so I wouldn’t worry—as I thought she was too young to be traveling abroad in such a career—she was not even a High School senior yet) normally (organized crime) doesn’t victimize innocent people, and, as with some countries in the world, may even be a better help to you if someone is kidnapped or disappears, as was the case somewhere else in the world I could mention (out of scope of this answer).
As far as petty thieves who steal from tourists and the like, I cannot speak to.