I have heard of anecdotal stories of increased incidence of meconium in the amniotic fluid, but that's the only really concerning thing I've heard. I'd never ever ever try castor oil again myself, though - I got to puke and poop all day when my midwife talked me into it, and still no labor until the next day, after everything had calmed down. I think I would rather have rested the day before the birth, yk? It was gross and awful!