I had looooots of mixed breed ducks over the course of my childhood. The only time they had trouble thriving was when I inbred one family too tightly trying to get a certain color quirk that the original drake had. I didn't know anything about genetics, obviously, but I did get that bib to come back. ;D My favorite colors/conformations came from Blue Swedes/Khakis, Pekins/domestic Mallards, and Black Runners/Khakis. There were a lot of colors I was unaware of at the time, though!
Wish I still had pictures, sigh. The first mix produced some nice medium-sized blue ducks with brown edging on their feathers and the usual white bibs. The second basically produced Black Swedish looking ducks with brown showing through in places - I was mostly impressed by them because I was NOT expecting that from the combo. LOL I'm pretty sure the mileage varies, especially in white ducks whose genetics are masking whatever's underneath. The third mix produced svelte black ducks with brown edging on each feather and unusually light-colored eyes for such dark ducks. They were gorgeous!