Really my only problem with the Cracks is that in The Eleventh Hour they seem to be some sort of portal or gateway, whereas then for the rest of the series they swallow things up and erase them. Not much consistency there.
It was established in Vampires of Venice that there are two different kinds of cracks, those which erase things, and those which act as portals. The vampire/fish arrived to Earth through a portal crack, though they were aware of the eraser cracks as well.