They can collide with objects on the ground too, causing damage to both people and their properties. There is always a possibility of the UAV going berserk due to loss of signal from the operator or some technical fault in the system. This will pose a risk of the UAV falling from the air and crashing in a populated area. This may lead to the debris entering the houses or people getting killed as a result of the direct impact.
This, however will depend on various factors such as, The population size The size of the debris The reliability of the vehicle History of failure of the device. System Reliability When it comes to relying on the devices and gadgets solely based on technology there is always an apprehension because of the history of failures that one has seen through the years.