Ducks are terrible protectors. Or, at least ours are. We had a fox problem last spring, guess which birds were the first to go? The ducks. And that's with two drakes in the flock. Our drakes could care less about what happens to the chickens. I've heard of aggressive drakes but I have yet to see one. A good rooster is the best protector for a flock of hens, if you ask me.
We recently acquired a goose and she is no alarm system. Unless you need an alarm to let you know when it's time to let them out for the day or to let you know when the dog is in the coop sneaking food I wouldn't count on a goose as being an alarm. But, she is still fairly new.