A flywheel can store so much energy. It is quiet limited, unless you spin it very very fast, then it might affect bike's handling (will be harder to lean). And you have to use it right away otherwise you loose it to friction. I guess it would be better to switch to an electrical system as Formula 1 did. That way you could add up all the energy from braking on a long descent and use it on the following ascend. But then it becomes an electric bike, so what's the point.