These are very cool, but I don't understand why they call them "bottom bracket dynamos". They're not directly powered by the turning of the axle at the bottom bracket. Why aren't they just called "rear tire (or wheel) dynamos"?

They are affordable for sure. I wonder what the weight difference between a normal hub plus this dynamo is compared to a Shimano 3N-80 or SON. I would assume the rolling resistance from a good quality hub dynamo would be less than this dynamo. Am I correct in that assumption? It would seem to be more prone to problems (misalignment, dirt/water, etc) too. Then there are the little things like unsightly wire to the headlight.