Yes unlike sheep/cattle you can't be rough with ducks cause they are more fragile. If a dog is too agressive and herd too close, ducks get all stressed out and panting. Obviously you don't want that. The dog has to give ducks enough room, at the same time you have to keep them together. When they are spooked they would spread out - making it difficult to keep eye on them.
Also If you own a smaller herding breed, sheep/cattles sometimes don't respect them and try to intimidate the dog by charging and head-butting. I talked to a lady whose Corgi did not pass the HIC with sheep for that reason. She passed with Ducks yesterday becasue ducks were far less intimidating for her. So it would be a good way to introduce herding too.
When Snowy attended Sheep Herding Clinic she was only 5 months old so she could not get a certification (has to be at least 6 months), but she did great - even though she was only 33 lbs puppy she instinctively knew to keep the herd together and sheep respected her. I will be testing her for HIC with sheep next month in Fowlerville, MI.