Flywheels have a lot of engineering hurdles that can't be met for $5: - they need to be well-lubricated, clear of dust (which I imagine to be a problem in some of the environments they're trying to sell to) - they need to be perfectly balanced, or the shifting center of gravity will vibrate the housing - sapping away precious energy. - they need to run at high, ever-decreasing speeds, which means generators that can extract a constant power level at different speeds