A Cooper's Hawk will go after your ducks. It's big enough to carry off a small to medium duck.
Guineas will alert to birds, dogs, fox, a bag blowing by, falling leaves, if you move something in the yard, if you look out a window at them, etc...
They SCREAM really really really LOUD! I don't mind it but if you have anyone close by they may mind. The girls talk 24-7 non-stop. I even hear them if I wake up in the night. I always feel bad for the boys having to listen to the non-stop coffee talk.
Guineas are bullies. Mine keep an eye on my bird feeders and will not let any other birds in the yard to eat. Even if I fill them with things the guineas do not want.
I got mine as keets and raised them in a cage in the duck pen so that they would learn that the ducks are part of the flock. It didn't work. As soon as they were full grown they started to attack the ducks on sight. I have to keep them seperate because they were terrorizing my ducks. It's been a few months since they were together and the ducks still run away at the sight of the guineas.
Now let me state that I love my guineas. They follow me around the yard and come running when I get home. They are hilarious to watch, do an amazing job cleaning up the bugs in the yard and garden. Also, I did have them kill a baby snake once. I only have three left of my original six because they like to do things such as fly into a tree hard enough to kill themselves, or into the side of my house hard enough to dent the siding and break their own neck, etc...
What I'm trying to say is that I LOVE guineas and will be getting more this spring, but they are not for everyone.