Sometimes, it is the people trying to be very conscientious and kind, that can ruin a broody hen. People tend to want 100% hatches and 100% live chicks, blaming the broody when they don't.
Then often times, for the 'best of reasons' people separate the broody hen from the flock. Then about 3-4 weeks, when the broody's hormones are falling, they put her back into the flock, where she has no status, no territorial rights, and is fighting for her own status, and chicks are on their own...again, a bad mother.
But if you just leave them alone. Every so often, some once a day, some every 3rd day, they get off the nest and go out to eat, dust bathe and terrorize the layers. I swear, once I heard the Rooster say, "Hey, Mother feathers, I AM the rooster!" But she paid him no mind! haha
Thing is she is on a hormone high, she stomps around, puffs up like a beach ball and puts the fear of God into the layers. They give her a lot of space, and they get in the habit of giving her the space.
Then when the chicks hatch, well the layers keep their distance (if there is enough space in the run). They get used to the puff balls, and while they may give a peck here or there. The chicks learn proper society (which means keep out of the layers reach, they are fast as lightening) and the broody hen for 3-9 weeks will protect them.
By a week old, the layers have accepted the chicks, and the flock is fine. Some broody's will keep their chicks for several weeks, and some will forget their chicks about 3-4 weeks. The chicks are on their own, but will do fine in the flock at that age.
So I have never had a bad broody, but I have had chicks die. Some were fail to thrive (a genetic disorder) some got pushed out of a nest, or stuck somewhere. I do keep an eye on them and try and prevent those, but it does happen.
I always hatch a broody. Even if it means selling or culling older birds.
Good luck,
Mrs K