What are the risks of turning a baby that is breech with a anterior placenta?

More difficult. The risk of causing an abnormality in the baby's heart rate is the same regardless of where the placenta is. An anterior placenta makes turning the baby a little more challenging because it is in between the doctor's hands and the baby.
Placenta seperation. I do not perform external versions on patients with an anterior placenta. The risk is that during the attempts to rotate the baby, the placenta may be seperated from the uterine attachment--a very big emergency. An anterior placenta also cushions the baby and makes it more difficult to rotate the baby.