Answer: Hello mate!
The advantages are:
- The dynamo does not require a power supply, which makes it cheaper, this is powered by the motion of the wheel.
- This is more environment-friendly because you don't need to change anything in the dynamo for it to stay functional
Disadvantages:
- Dynamos can store some energy, but in general, you need to move if you want to activate the light, if you stop, also the light. Also, a change in your velocity can imply a change in the intensity of the light, which also means that there is a minimum velocity needed to turn on the light.
- You may find a little more resistance when you pedal, this is because the dynamo is applying resistance (by friction) in the wheel.