Yes. Here are some examples:

1. they distract the pilot during the most critical phase of flight

2. In an airport environment, planes are closer to each other than elsewhere in space, so if a pilot has to suddenly deviate to miss a drone, he might hit another plane

3. If a drone gets in the way of one side of a LOC or a GS signal, it might mess up the plane's ILS approach, possibly causing it to crash. Birds (being radio-transparent) do not.