cheap to make - if you care to do so -- to stop them your enemy has to deploy expensive anti-missile tech -- thereby in a war of attrition the person who quickly churns out drones wins.
drones can also destroy -- more expensive sites like oil plants, factories etc that cost XXX more than the drones
and the person who flies the drone if not automated -- can be safe somewhere
The Gulf War era world, where a small professional force using expensive high-tech gadgets could win a war without the enemy being able to fight back, is gone. Today you still need the small high-tech force to win, but you also need a larger cost-effective force for defense.
A guy got close enough to shoot at and almost kill a Presidential candidate during the last election -- imagine how different things would have been if he had just used something like this[0] instead.
The first time someone attacks an outdoor music festival with drone strikes like that asshole who shot up that concert in Vegas is going to change society for the worse. :-/
[0] https://www.youtube.com/watch?v=qrZ1aH5gtMU&pp=ygUVd29ybGQnc...
That particular attack against Russian bombers was done by undercover saboteurs working deep inside enemy territory. The drones being discussed here are essentially cheap cruise missiles. For example, you have a ~200 kg drone carrying a ~50 kg warhead that is launched using a rocket booster and capable of flying 1000+ km. In Ukraine and Russia, 80-90% of such drones are routinely intercepted by cost-effective air defenses.
The US military has used microphone arrays and radar as well to determine the trajectories of local threats, from snipers to field artillery.
Probably next would be "lasers" to disable any threats.
Styropyro and Tech Ingredients both have had youtube videos on high powered lasers.