Escort order (white line) and 'hold' will prevent drones from firing.

They also have (more generally) an issue where they are slow to perceive an incoming threat. This is probably because they check for hostiles every few ticks in a conservative manner.

Last but not least, they never retarget biters when they are already set on a target. For example they start firing on a nest, and they will die against spawned biters because they are still not done with the nest.

So all in all, they are nice, but implementation is rather rough.