I think it depends on your veg patch.
We have things growing in a polytunnel and 2 foot high raised beds. The ducks get to patrol all around the area, but not actually inside the polytunnel or on top of the beds, so no actual contact with the veggies.
I've definitely noticed a reduction in the slug population since we unleashed the ducks, and no downside that I can see. If you don't have raised beds, perhaps you could fence them out of the actual veg rows, for a similar effect?