What are the primary challenges for genetic modification in adult subjects?

My understanding is that we have several tools which could allow for significant genetic modification in embryos, but that there are legal and ethical hurdles. Now, most of the ethical hurdles would go away if you had an adult human who could consent to the procedure. (And also, I am not an embryo and would dearly like some better genes.) So, what are the primary barriers between where we are now and being able to modify adult subjects? (By "adult" I mean "born" -- I suspect children would be fairly similar, although probably easier than adults since they haven't fully grown yet.)