If your placenta doesn't move and stays by the cervix then you'll likely have to have a c section. Other than that it usually doesn't cause many complications.
I had placenta prévia with my previous pregnancy. I did the worst thing possible and googled it so then I was terrified. My placenta finally moved up around week 30. I think the biggest concern is that if the placenta doesn't move up you have to have a c section because the baby won't be able to make his way out of the birth canal if the placenta is blocking it. Also, in some cases bleeding can occur.