I've had many bronchs. So many, I can't remember them all!
We did have a negative consequence from one bronch, but all-in-all, they have been a very necessary tool for diagnosis & maintenance.
Like any surgical procedure, there is some risk involved.
Honestly, I wouldn't worry too much; as long as you have a competent doctor performing the procedure.
Do you trust your son's CF team?
Feel free to discuss any & all apprehensions about the bronch with your son's doctors.
Remember, the only dumb question is the one you didn't ask.
Best of luck to you & your son, I hope everything turns out for the best.