Something really graphic might not be allowed at all. Something questionable might be allowed to be posted but might have advertisements disabled. I for instance wrote a hub on sexual education in public schools. It was fine to be posted but ads on it have been disabled. And I think it is pretty mild - it mentions a couple of questionable words I guess but doesn't in any way describe them.
My sense is that hubpages will allow above PG-13 but not all the way to R, while adsense wants things to be under PG-13 (or maybe even under PG). maybe that's a more helpful way of putting it.