Drones "mutate" into the buildings they construct. The only way to stop a building from completing is to destroy it or to cancel it; in the latter event the drone is recovered and may be used for some other purpose. Once the building is complete the drone is deceased.
Drones benefit from burrow. Unless the enemy brings detectors burrowing can minimize the damage caused by a raid on harvesting operations. Once the enemy is repelled harvesting may immediately resume without requiring the replacement of all workers.