Hacking planes
I am not a hacker but I am a software engineer and I am aware of the generalities.
You need something common like MS Windows with thousands of people having a go at it from many angles. Work is not done by spotty teenagers but by quite seriously skilled people who have experience and create tools. The teenagers usually use tools made by others.
Many attacks succeed because doors are left wide open for them - e.g the practise of people having administrator privileges by default means that any program you launch can start modifying the system. This is pretty basic but it has been the case for years with older versions of Windows for reasons of ease of use. Other attacks happen because known flaws are not corrected on all computers and if you simply try 1000 old tricks you eventually find one that works.
It is a lot harder to attack an unknown and rare system (surely the rq-170 doesn't run Windows?) and harder still to do it when you only have one article to attack for some limited time period.
I am more inclined to think of attacks involving jamming or signal corruption or something like that (possibly because I don't understand that area well).
Most complex systems cannot be bug free and bugs often lurk in error-handling routines because people almost never manage to test all the possible combinations of mishaps. I am wondering what drones do when something goes wrong with their communications. I can imagine triggering a failure by luck.
Espionage would help that luck a lot, of course.