To answer your question, no we haven't replaced a ground sprayer with a drone. However, I do try to stay somewhat in the loop on drones.

Drones seem to be working well. But the non-aerial label of many products could be a problem. 

Have you checked w/your insurance re. drone drift? Will that be a problem for you? How about the necessary licenses? 3rd class physical needed for operator for drone over 55 lbs? Yes, I'm aware of how life really is also.

One minor problem I see for drones is how rapidly a model can become obsolete. Not a big deal, but perhaps needed updates won't be available? I don't think we see this in ground sprayers. A 10 yr old JD sprayer is just as functional as a 1 yr old one. I'm not sure there is a 10 yr old drone sprayer yet. You could spend $50,000 on a drone & it's batteries now, but how long will it be serviceable? I'm not sure, just asking. I also look at the total cost for batteries to be included with the drones. 

Drones can & do crash. The same doesn't happen real often to ground sprayers. There are probably 50 times as many ground sprayers as drones? Maybe more, so the exposure rate to ground sprayers should be more, but they just don't crash often.