so you’re claiming he nailed it?
Drove it and left a waffle mark! 
Check out the new guy, hasn’t even been around long enough to wear the milling off his hammer face 
that’s not a roofing hammer.
They could be doing all of the roofing by using hand tools. No air tubes, no battery replacements. Much quieter.
But it takes much, much longer to complete. Speed and efficiency with low-cost unskilled labor vs. time, money and accuracy with those who know how to use a roofing hammer correctly.
Pneumatic nailers also work at any distance from the compressor. The air for each nail is stored in the gun. Electric nailers would be limited by the amount of current delivered to the tool, meaning that the carpenter will need to use oversize extension cords or will have to work a limited distance from the generator.
Also, the standard 110V outlet is rated to provide less than 2-1/2 horsepower. An air compressor may be able to produce more continuous power, and the power can accumulate between each nail.