I've used it before for both ducks and geese (geese are hard to worm by mixing in with food because they eat grass) and nobody died! The only hassle was that we were told we had to give it orally, three days in a row, which wasn't the easiest task with half a dozen identical looking ducks. We found the best method for was for one person to hold the duck and open its beak, whilst a second person put a syringe right into the duck's mouth and squirted it down over the back of its tongue.
There was also some considerable debate as to the correct dosage for poultry, so make sure your vet looks it up. I'd definitely use it again though.
BTW, when I last checked, it was cheaper to buy the layers pellets with Flubenvet pre-mixed than it was to buy a tub containing the same quantity of flubenvet on its own. HTH!