Unfortunately, current LED lights aren't bright enough to illuminate the road, so you need at least one incandescent bulb facing forward. Generators get energy when pressed against the spinning tire. Generators can be disengaged when you don't need any light. A generator adds drag and slows you down, although the good ones don't slow you much. Generators don't get a good grip on some tires, especially when the tires are wet.
Generators require more skill and patience to install than do most battery-powered lights.
Even when new, none of the generator lights I've owned have been as bright as I would like. Yet, despite the many disadvantages, I like generators a lot. Unlike batteries, they rarely go dead when you're halfway home. And generators are more environmentally friendly than disposable batteries. A light that you can mount on your head is more flexible because you can point it whichever way you want.