My goose is a wonderful protector of his flock, always on guard. I agree with the sentiment that they can't be considered the sole line of defense--they will quickly fall before any determined medium- to large- predator. But they provide two important services that can serve as PART of an overall protection plan:
* Early warning: My goose keeps a close eye on the flock and everything around, and when he sees something disturbing, he does not hesitate to call it out, very LOUDLY. When he makes a racket, I come running and what he can't protect the flock from, I can.
* Deterrent: While a goose can't defend against a determined predator, most of the time most predators are looking for the easiest meal, and a flock protected by a goose may not make the best target. So while it's not 100% protection, it is fairly decent much of the time. There is much to be said for ATTITUDE.
I still keep my flock locked up at night, and I did lose a goose to a domestic dog (grrrr), but for the purpose of deterring a pestering hawk, a goose will probably do the trick.
My goose also is much more intelligent than the ducks and thinks on their behalf. One morning he brought one of my hens up from the lake so we could untangle her from some fishing line. He then got her settled in the pen and went back to the lake to bring the rest of the flock up.
My only concern is that if the goose is not raised with the ducks, he may not identify them as worthy of protection. Mine were raised together, so he thinks they're his family. He does, however, accept new additions to the flock once they are full grown--as though he somehow knows all ducks are potential family, having been raised with them.
Good luck. I love geese for their own sake and I do believe they are a valuable addition to a flock.