After seeing a ballast spreader on this forum I decided to make my own; I cannot share the design as it MIGHT be a patent issue; even though my design is different and has nothing to do with the other ones.
Here is the problem with a spreader machine: 1) to hold enough ballast to be useful it must have a large ballast container; 2) if it's that large it wont fit under any bridges or near any vertical supports that are too close to the track being ballasted. Further, if using Ross switches, take care as you don't want ballast to go in places that may impede the switch or connecting rods or getting small dust into switch machines.
So any machine has limits on usage; hand ballasting seem a better way to go.