I think the 5 out of 7 has to do with the limited amount of hairs these trial patients probably recieved. It was, afterall, to assess the potential health threats, so i'm sure a very limited amount of hairs were implanted. As long as this procedure proves to be safe, and cancer ruled out, I don't see what's to stop it from coming out within the next several years. But man would it be the biggest kick in the balls to find out HM surgery has serious potential side effects.
Even if the hair grows in retarded directions, using HM with 4000 or doner hairs has got to produce some pretty awesome results. This sh*t's gona cost more than luxery car though, at least that's my guess. Especially if they have to use HM and regular transplant surgery together.