Maybe my parameters are skewed. IMHO sketchy areas in London, Paris, Milan are more threatening than anything I've seen in SF peninsula, despite the obvious difference in guns per capita.
Can't speak to Milan but I've never felt unsafe in Paris, and only once felt unsafe in London (and that was at 3AM and I was dressed... interestingly), whereas in the space of a week in SF I had two scary encounters with ranting homeless guys.
If you ignore the crime problem in crime-heavy areas, you might say that those areas have no crime problem. It doesn't mean that you're right.