I read one site which said they can and will produce mules, but I've never seen anything resembling a cross between a duck and a goose before. Wouldn't it be cool if someone tried to do that? I know there's at least one person who has a goose-Muscovy pair.
ETA: I know of instances where ducks and geese have mated and had no offspring. So, no, I don't really think it would ever happen. I don't know why.
I think the comb duck and white-winged duck were bred with captive Asian Muscovies. They said the resulting offspring was mostly sterile, except for about 1 in 30 males and these fertile males helped create more domestic Muscovies. But, then again, I heard that Muscovies were tamed in Mexico before the Europeans arrived, so I don't know how credible this information is.