I know they can mate and have been told by others that theirs have done so, but I've never seen it happen successfully with my birds. There is offspring around as proof: they call them mulelards or mule ducks because the offspring between a mallard-derived breed and Muscovies are sterile.
I hear that if you do let them cross, the best combo is between Muscovy males and Peking females if you want an amazing meat bird that gets heavy fast, or a Khaki or Runner female if you want a hybrid that lays a lot of eggs. Something to do with hybrid vigor. Oddly, using a mallard-derived breed as a sire (the drake) and a Muscovy female leads less robust or poorly performing ducks I've heard but this is something I was told and have no experience with.
My Muscovy males tried to mate my Khaki female, but it just looked so ... uncomfortable for her that I separated them. She's my princess and I couldn't bear those hulking brutes going after her so I hunted down someone with a Khaki male her age. The Khaki drake has been going after my Muscovy females as well, but they aren't ready to breed so he's been rough with them in pulling out their neck feathers so for now it's just him and my baby all alone, but I hope someday to be able to try a cross.