Prepackaged lights for cycling seem to be way behind the times. Not that I blame them, the pace of the fiber optics industry seems like light speed compared to the cycle industry and rightfully so, afterall, unlike cycling fiber optics is a very profitable business.
Although I haven't built any of my own lights yet, my impression is that the biggest "disadvantage" of doing so is that it's difficult to construct durable optics on par with what you get from prepackaged products. I put "disadvantage" in quotes because, frankly, I think it's a good thing to have extra light diffusing in all directions for safety reasons.
Maybe this website will be helpful:
http://pilom.com/BicycleElectronics/DynamoCircuits.htm