I used to know a flight purser with an international airline who mentioned that they had certain international journeys where they didn't overnight due to security concerns.

Other cities where they had to overnight, they would be forced to stay in a secure hotel and would not be allowed to leave the premises.

I find this whole idea pretty interesting.

Does anyone know of what's perceived by airlines as rather more "risky" cities for their crew?

I would assume this varies by airline to airline, depending on their cultural/political relationship to certain regions as well?