If I remember correctly I think that the ballast doors not only rotate inside/outside of the rail independently and then they also have a lever that controls the amount of ballast that dumps. The MOW crew sticks a pry bar into the gate to control the flow. The key to dumping ballast is not so much on the MOW crew, but the hoghead running the train. If the hoghead goes to fast or to slow ballast doesn't get dumped right.
If you ever look at a newly ballasted line you can notice the unevenness of ballast dumping. When I worked a work train we dumped ballast for 30+ miles one day, and the foreman in charge kept yelling at me to slow down, speed up, slow down, speed up, etc. The next day when we went over the tracks were we had dumped I noticed big piles of ballast in certain areas, and lightly ballast in other areas. Oops! My bad!