Look at the top fuel bikes, they use chains. For high powered machines the chain and sprockets are the most efficient drive, for lower output bikes a belt drive is a little better.
Shaft drives are for longevity and low maintenance. They almost never break or need replaced.
The drawbacks are several. Weight, cost, handling and efficiency. Notice that the motorcycles that are shaft driven are usually longitudinal engines, like a BMW Boxer or a Honda Goldwing. The crankshafts rotate in the right plane for a shaft drive. At the rear there is a gearset that changes the rotation 90 degrees to drive the wheel. Each time you pass through a 90 degree gearset, you lose a bit of power. With an engine like a CG you'd have to have two 90 degree gearsets, one front and one rear.
If you have a suspension in the rear, the shaft will make it stiffen up under acceleration and droop on trailing throttle.
A good toothed belt would be best on a CG, but the cost and inability to have a standard length belt in the kits make it a non-starter. You can't take a few links off a belt.
The chain rules, for now anyway.
I would say that once it is aligned and tensioned right, on these bikes they are just about trouble free. I don't even worry about it jumping off, it's smooth and quiet as a belt, almost.