Grid fins are used "commonly" in aerospace - usually for things like control of guided bombs or spacecraft. The most obvious example is SpaceX's use on the Falcon 9, but the US and Russian militaries have been using them for decades.
My understanding is that there's a tradeoff with grid fins. They offer greater control authority (and/or lift) in a more compact space, and require less torque to turn/reposition than the equivalent wing would. The downside is that they have a lot more drag, and are almost useless at trans-sonic speeds.
My question is: Why aren't they used as wings (to produce lift) on low-speed aircraft and RC drones? I was unable to find any examples of grid fins being used as wings or lifting surfaces. It seems like if it were possible to create more lift in less space someone would have used it in a design. Am I missing something obvious here? I'm guessing yes. :)