Simple answer, it depends who you ask!!!!!!!
When we looked into this, a local chicken breeder would not sell us hens as we had ducks! Ducks are very messy, they will poop anywhere and everywhere, including in their own food! And hens tend to be a little more fussy!
The biggest problem that we could think of was the ducks swimming water - not all hens are all that bright, and if they wander into the duck's pond, they can easily drown, as their feathers are not waterproof, and they can get dragged down by the weight of soggy feathers - but this depends on your pond set up. our ducks never slept in doors, and therefore there may be a way to seperate up the duck end of the run from the hens end?
Kathi