Broodies have been raising chicks with the flock for thousands of years. There are risks whether you raise them with the flock or if you isolate them. I personally prefer to have the broody take care of the integration issues for me. I consider that the less risky option.
Occasionally you will have a chicken in the flock, usually a hen, that will try to kill the chicks. She will seek out ot destroy the weaker chickens. Most flocks don't have these, but some do. Usually the broody will teach a hen that threatens her chicks a severe lesson. But not all broodies will. They are living animals so you really can't tell what any specific individual will do.
I've had two-week-old chicks line up next to adults to eat or drink. Sometimes the adults ignore the chicks. Sometimes a hen will peck the chick to teach it that it is bad manners for an inferior chick to be in the personal space of a dominant chicken. The chick goes running back to Mama as fast as its legs and little wings can move it. Mama usually ignores this. It takes a flock to teach a chick proper chicken etiquette. If the hen is trying to hurt the chick, then Mama does take major offense.
The pecks the chicks get in these circumstances are not meant to kill. They are meant to discipline. As long as the chick runs away, proper chicken etiquette has been maintained and all is well. If the chick does not run away, then the adult wil get violent. Same thing happens when you integrate older chickens. The grown hen is a lot bigger than a small chick, and it is possible the chick can be injured or even killed by these pecks, but I've had several chicks pecked like this and none have ever died from it.
A chicken flock has its own rules of order and its own dynamics. As long as evey chicken understands its place in the pecking order, the flock is peaceful. And the flock has its ways of establishing and maintaining order.
The only time I've lost a chick was when one got into a pen of 8 week olds and Mama could not get in to protect it. If Mama can protect the chick, she usually does a real good job of taking care of her babies. And usually, the other chickens are not actively trying to hurt the babies.
