Coordinating movements and responding quickly in an intelligent manner are not trivial tasks. All manner of countermeasure could be developed. Some of them would be really cheap.
It is one thing to win a war with drone swarm, but may be another to effectively enforce occupation.
You are looking at a very large scale combat operation which was not the intent of this post nor could I even begin to engage in this topic. Can we reign this discussion back to possible uses by local law enforcement as a tool for first responders?