The issue with them is that they can be weaponized... if they're strong enough to carry a payload, that payload could be a bomb... so the FAA doesn't allow a payload any bigger than "X" (not sure what the limit is, but it's pretty small). And if you can't have a bigger payload, you're pretty limited in how much you can get done. Yes, it can do it, but the reloading time gets pretty limiting. And of course, the bigger the payload, the more energy it takes to lift and fly and apply... so more battery, more props, more structure, more... more... more...
So THAT limitation then dictates how much water you can reasonably apply per acre, which greatly limits your options for chemicals, and their effectiveness. They cut the rates, not because of how effective the chemical might be at that rate, but because of how unnacceptable it would be to have to put on more water and chemical.
Which of course contributes to the increasing evidence of chemical resistant weeds... the ones that make it with the lighter rate are the ones that are more resistant. That's what you get for a seedbank then. AND, there is evidence that not only do these resistant varieties survive, they are also ENHANCED, and make MORE seed, after application of a herbicide they have some resistance to.
Low-dose herbicide applications can accelerate resistance - WeedSmart