An SR-71 would be a hard RC plane to fly. I read a really good book called Skunk Works that talked about the development of that airplane. That planes aerodynamics are not exactly designed to nicely fly at low speeds - it has to land fast because of the shape needed to be stealth and fly at high mach speeds...although I guess making an RC Citation jet isn't as cool as an RC SR-71.
So, no knocks there, just sayin' hehe.