The only advantage of the starter is that you just push the button and it starts right up. For a one man team that may be a nice feature. I don’t like to have that extra weight on the kart especially hanging on the motor but to each his own. Some people do like it on the motor because that have to make weight.
The ring gear does take some HP away from the engine since it acts like a fan. It’s similar to old cars with the fan on the front of the engine. It takes HP to turn them.