I see two evident problems with directly feeding ducks snails on a daily basis. One being, is snails and worms often contain a generous amount of internal parasites, so you are technically increasing the chance of your ducks coming down with an overload of worms by feeding it to them directly. Of course, they'll likely eat them anyway, but "reducing" the amount in their diet should help.
The other problem, I see, and something we see yearly is the risk of choking on the snails. Due to the slimy nature of the snails, they can easily get stuck in a ducks throat and cause asphyxiation.