Here's a hypothetical matchup for you. A column of tanks covered in thick armor, capable of firing many bullets of both large and small caliber. On the other side, a bunch of plastic quadcopters you bought at the local Walmart. Who wins? According to exercise by the US military, the drones have it hands down.
Drones > Tanks
This was one of several fascinating factoids from a Wired report on a recent meeting hosted by the Department of Homeland Security, the goal of which was to access just how dangerous these "toys" can be.
Along with DHS and the US military, the Federal Aviation Administration was in attendance. That agency has a mandate from Congress to come up with new rules governing domestic drones by the end of this year. So far the agency has basically delivered a ¯\_(ツ)_/¯ and some delightfully low budget YouTube PSAs.
Other fascinating factoids from this meeting include:
1) Syrian rebels are importing consumer-grade drones to launch attacks
2) A DJI Phantom 2 can carry three pounds of inert explosive. Or at least you can strap that to one sitting on a table and make it seem terrifying
3) The software DJI recently used to try and prevent drones from flying over the White House was originally developed to keep the craft out of China's Tiananmen Square Square
4) The military has attached automatic weapons to cheap commercial drones and fired them with great success
The reassurance offered by the Wired piece is that it's actually pretty easy to jam the radio signals which are used to pilot these drones. As several commenters point out, preventing a drone on autopilot from flying a route mapped by its GPS might prove to be a lot more difficult.