Its possible a sleeper cell of terrorists could effectuate some small area drone strikes with commercial off the shelf drones and improvised explosives.

The large scale military drones you are envisioning that can do the same damage as military aerial bombardment, that is a much harder thing to “sneak” into the US at any kind of scale or to build in secret.

As for future state actor capabilities. It seems possible that China is working on drone tech deployed from submarines or other force-projection platforms. Yet another reason to avoid a hot war with near peer militaries in current year.