You are more likely to heamorrage with a PP as where the placenta lies sometimes its neccessary to cut into a part of it to get the baby out. This obviously needs to be done super quick. It is rare for this to happen. Also as the lower segment of the uterus is not the area that is designed to contract the blood vessels in the uterus that have being feeding the placenta do not get constricted as well as they would be if the placenta wasn't in the lower segment of the uterus. The fundus (top) is the area that contracts the best and is most efficent in reducing the size of the blood vessels from the placental site.

Most PP that are grade 3 or 4 are done under general anethetic I guess more for prevention then anything else if something was to arise during the procedure. To my knowledge the worst PP where the placenta completely covers the whole cervix often deliver in the late/early third trimester. I have only seen one deliver at 36weeks the rest around the 26-30weeks.