Adult stem cells can only form certain kinds of other cells, and thus aren't super useful unless you can find just the ones you're looking for, and lots of them. Embryonic stem cells can give rise to a much wider range of cells and, if you have a good way of growing them outside the body, you can make tons of them. The reason this new method is safer is because it doesn't involve inserting new genes *into the genome*, where it could disrupt important functions (this has happened in the past, for instance, with the gene therapy treatment for X-SCID, which lead to several cases of leukemia). In the new method the four introduced genes are on an extrachromosomal cassette which doesn't integrate into the genome (and eventually degrades), so there's not as much worry about disrupting, say, a tumor-suppressor gene. Of course, that's not to say it's completely safe....