Drones themselves do not undermine democracy. If they have come to be used without much political debate, that is not the fault of the drones themselves. If it is a problem for democracy, the problem lies not with drones but with Congress and the people who do not choose to make an issue of the use of the drones.
I am not surprised that there has been little controversy over the use of drones. After all, they are just another weapon and not even a weapon of mass destruction. It may well be that we should be having a debate about how they are used, but that is, again, not the drones' fault. So drones do not undermine democracy. If democracy is being undermined, it is people who are undermining it by not talking about the drones.