Have replied about this before but lost the reference. In essence, the risk of placenta praevia and accreta increase with each section, so that by the 4th the risk has increased to enough to worry us anaesthetists enough to get 4 units of blood crossmatched and the rapid infuser ready before we start the op.
Placenta praevia and accreta are where the placenta implants into the scar, and doesnt come away, so mother bleeds. There are degrees of severity, some are not too bad, some dreadful and would require hysterectomy and other surgery to sort out.
However, if your placenta on scan is away from the scar, it is probably ok, and you are not at risk of that particular complication.
There are other risks, as nina pointed out increasing surgical difficulty, and of course each time you have a section you run the risks of wound infection, and DVT/PE, particularly if overweight.