My personal experiences....
Ducks are very happy to be in the same pen with chickens, but they foul the water for everyone. You'll be changing it out every day, which is a pain in the tush and muddies up the whole yard.
Ducks lay a lot of big eggs. If you like duck eggs (I don't), that's great. If not, what are you going to do with them?? Last year I just tossed them into the chicken yard and let the chickens eat them.
Ducks eat a lot if they're cooped up.
Most ducks are NOT cuddly, friendly little creatures by nature. You have to handle them multiple times a day from hatching on if you want them to be pets. Mine still act skiddish if I get closer than a foot away.
If ducks aren't cooped up, they quickly become dinner. We lost every breed of duck we ever bought to coons, possums, foxes, and coyotes. Except the mallards, which can fly. I noticed the neighbors have had good luck with muscovy, but they also fly.
The cutest ducks are crested. So I'd suggest buying crested Mallards if you're going to get some. Ideal carries them.
Kathy in Texas