I have seen my guineas chase off a fox, a raccoon and a coyote! That was during the day. It is quite funny to watch because the entire flock starts their alarm call and then runs at the offender with wings flapping. Any of those predators could easily kill the guinea, but I guess they are so freaked out when 25 flapping and squawking birds come at them, they just run. They also alarm on snakes. And they keep a good eye out for hawks and alarm on those as well. I think they help my chickens since I don't have an adult rooster yet. Never seen the guineas actually kill something larger than a grasshopper however. The chickens seem to be better killers, but that's just frogs, lizards and mice.
The guineas that don't come in to the coop at night do not fair well at my place. We have coyote, bobcat, mountain lion, owls, raccoons, ring tail cats any of which can get guineas even when they roost in the trees. So I try my best to get them all in before sundown.
I start letting mine out of the coop after 2 or three months.