Inefficient compared to a B-29 dropping a bomb over a city but not inefficient compared to whatever other means (if any) Germany had of attacking an east cost city like New York.

Rest assured that if this alternate history scenario had occurred historians would not be talking about the inefficiency of the attack, they would be talking about the most devastating attack that Germany was able to conduct against Americans on their own soil during the war. It would have completely changed the American mindset and culture forever.