They will hiss and let me know if something serious is out there creeping around. Mine spend the night outside of my trailer here, I swear they like to listen to the radio, but in reality it's the light and I think they feel safe around me. There's a little alarm call they give but when there's a possum or something around they' ll hiss and it's not their normal hiss, they mean it.
They are blustering bullies and if you don't know they're the biggest wusses in the world, you can indeed train them to be "bad birds" who will chase, bite, etc. To me my 14 geese are wuss-covered with a creamy wuss center but they chased a friend of ours, a guy much bigger than me. Too bad I didn't get to see it, I'd have laughed so hard!
This place is 5 acres and it's a pretty "busy" 5 acres, it's a lot to expect geese to patrol. It's more than a lot of dogs could patrol.
They will pick on smaller birds, we had chickens just kind of running wild here and of course they wanted to take part when I was giving the geese their corn, and the geese would try to grab the chicken, naturally I thought this was hilarious and would tell them, "get that chicken"
For a smaller place, maybe an acre or two and less clutter, they could be really good watch-birds.