Agreed. I did a napkin-back calculation on a giant (50m diameter) grid-scale flywheel back in grad school in '95 (when there was talk of powering cars with them) and failures would have been catastrophic since the flywheel would dissipate all of the energy nearly immediately (in comparison to gasoline). A car powered by a flywheel (0.5m diameter) would have been a guided missile... Fortunately, grid-scale flywheels can be placed in out-of-the-way locations.