At first shooting at it did nothing. Then when it was "fixed", I would see a message saying it was destroyed and see the explosion image, but it would keep on trucking, occasionally staying around for the entire map (including multiple rounds).
When working as expected, and being able to be shot down, I think 4 kills was fair for what it did. You can get the same number of people defrosted with a car (distance permitting) and that one is only 3 kills. I would say shooting at the uav is much easier to destroy than trying to kill cars (especially with hardwired perk).
I would love to see the uav defroster back!