Ummm.... yeah, they're good fly catchers--but you're not thinking of making them catch all their own food, are you? Ducks, like any domesticated animal, have significant care requirements. They need predator protection and they need a good quality feed, plenty of water, and shelter in the winter. You also need to be prepared for waste management if you have very many in a limited area.
Sorry if I read your question wrong.
They are quite good at catching all kinds of critters--flies, moths, butterflies, mosquitoes, gnats, japanese beetles, etc. The problem with the flies, I have found, is that their poop creates such a wonderful breeding ground that no matter how many flies they eat, having the ducks means we have more flies than we did before.
Good luck whatever you decide!