Some people hire a mohel based on word of mouth. Their friends or relatives had a good experience, and so they are confident they'll have the same.
Some people hire a mohel based on their own homework and research, and interview several mohels before deciding which they'll hire.
In either case, everyone wants a smooth experience, and a perfect bris outcome.
Sometimes that happens! But we are humans, which means by definition we are imperfect.
Not everything is meant to be perfect. And since every baby's anatomical situation is a little different, there are no guarantees of perfection in life.
In some cases the precise amount of foreskin is removed, the perfect amount of membrane remains, the baby's bleeding is under control within seconds, and the care he needs is minimal in the days to follow. This is about as close to perfect as possible.