Ducks are incredibly messy compared to chickens. They throw water everywhere, and need to have deeper water to wash their noses in. They also need to bathe, like in a baby pool (hard plastic type) so that their oil gland doesn't clog up. I keep mine separate, to be sure the chickens have clean water at all times, and that the ducks can drench the place without offending the chickens.
I built a drinking station for the ducks, to keep the bedding dry. It's a wood frame around a plastic tote, with hardware cloth above it. Set the water in it, and all the splash drops through the wire into the pan. I need to empty it about every 3 days from a 3 gallon drinker. Otherwise, I would be mucking out the duck side daily to prevent mold and stink.
Outside they have a baby pool, pine needle bedding, over pea gravel. The chickens have sand, covered over with pine shavings.
Also, drakes can severely harm or even kill a hen. Fun fact of the day, drakes have a penis while roosters do not. If a drake tries to mate a chicken hen, he could really hurt her. Drakes tend to be jerks during mating season.
Unless they're raised together, they won't ever really be "friends", they'll have sub flocks and bickering between the groups unless they have a lot of free range space.
The bickering is different too. A duck will snake it's head out and grab, or muscle another bird around with their body. Chickens chest bump. Today I watched Hammie (who was raised with ducks) get into with my 9 pound Drake Earl. He was running because she kept after him. But if he decided to get back at her, he'd grab hold and not let go right away. She's asking for trouble, another reason I keep them separate. The other chickens are afraid of the ducks, they didn't grow up with them.
With limited space, they don't always mix that well. A lot of people have no issues though. I just prefer to be safe rather than sorry, and with Hammie how she is, and Earl how he is, combined with the mess and water.. separate works best for us.