I have ducks - they don't nest in raised coops, they do it on the ground. They are MESSY. Disgusting, dirty creatures. Will instantly soil your water sources with sand/mud/dirt/whatever (part of their eating process, actually) which is going to increase maintenance requirements on your part.
Mine are Pekins - meat ducks. Fast growth, good weight if you feed them. If you free range them, as I do, with supplimental feeding, those things are much slower. Ducks have "dates" when its best to process them, to ensure a cleaner carcass, which takes a little planning on your part and requires you have the schedule to accomodate. Also, for all that they are poultry like chicken, the carcass is just different enough to make it annoying to break them down until you get used to the anatomical changes.
Apart from that, they are really tasty - not sure what kind of demand you might have - but they are tasty. Eggs are huge, very rich, but they are slow to lay (around 7 mo to start) and infrequent at it. I've most of two handfuls of hens right now, some won't be of age till almost October, and I've not seen eggs in weeks. Eggs in nests which they alsomight be hiding somewhere in the pasture.
Oh, and its a 4 week incubation, not 3 - so if you only run one incubator (as I do), it can create real bumps in production routines. 4 weeks for a duck hatch, 3 weeks for a chick hatch, a couple of days to account for stragglers and clean the incubator, suddenly you have a two month period with no new egg layers coming on line...