Agreed, above trees and buildings there is a lot less air traffic to worry about. But you get into the inherent dangers of air travel. Helicopters are especially dangerous, unlike planes if they lose power they cannot glide at all. In addition they take off vertically, assuming there will be set takeoff landing areas, checking for rapidly ascending and descending aircraft will be very important. Birds are always a concern when it comes to propellers too. And if used in a city up and down drafts created by large buildings like skyscrapers will provide a large controls problem, let's hope those controllers can reliably handle impulse forces.