I've had guineas over 3 years now and they free range, covering 25 to 30 acres every day. We have all kinds of hawks and eagles but have never lost a guinea. I've seen mine get in a tight group and honk VERY loudly and it seems to scare away the predator birds. And they've done it with dogs too. The most important thing is having them trained to sleep in a coop at night and shut the doors. If they roost in trees they are very vulnerable to owls and other predators.
On another note, We used to kill rattlesnakes on our property every year. Our neighbors still do. But since getting guineas I've not seen a snake around here. Or mice for that matter. The grasshopper plague in our pasture has disappeared too.