The nature of ball milling makes it a very inefficient flywheel:

The energy of the wheel is going into repeatedly lifting the material. That energy is lost: it's not transferring that energy back into the rotational energy of the wheel every time it falls, but into breaking up the material against the bottom of the wheel, more or less perpendicularly to the direction of the wheel's travel at that point.

Flywheel are efficient for high power in short duration: https://en.wikipedia.org/wiki/Flywheel_energy_storage

Usually used during a short period of time before the diesel generator can be fully operational.

Maybe they can put these near power plants and use for power factor correction.