Do you have any references that uav can move a mass with less energy use or pollution? As far as I know they would use more. flight can be more efficient but usually only if its a long distance because take off and landing is energy intensive. Drones fly somewhat like helicopters though which I was under the impression was one of the most energy intensive travel means. as far as I know, good to bad wise, its like walk - bike - train - bus - long flight - car - short flight - ships (mainly I think do to how they are powered. sailing or electrified may be way better) - and helicopter would be at the worst level of pollution.