It depends on the size of the drone and where it hits.
I am not sure how large these drones are, but if they have the mass of a flock of large seagulls, say, it could disable an engine (like the US Airways jet).
A collision with a wing could cause substantial damage to the wing which would change the aerodynamics, and that could be potentially dangerous at low altitudes because of the neighborhoods underneath. Debris could fall from the sky and injure or kill someone. I doubt it would come close to tearing off a wing.
A collision with the fuselage could breach the cabin which is dangerous in itself, but would not cause a decompression at that altitude.
A collision with the windscreen of the flight deck would be dangerous for obvious reasons. In the minor case, it can blind the pilots temporarily. In the worst case, debris from the drone would incapacitate the pilot during the most dangerous phase of flight: landing or departure.
What is really dangerous is the speed at which the aircraft hits the mass of the drone. It may seem like the aircraft would just kill the drone, and that is probably the case with most drones, but if it has large mass, it wouldn't.
The most dangerous scenario is if a drone hits the control surface like the flaps (very important for landing), or the horizontal stabilizer in the tail, it could be disastrous.