Meh... IMO everything that requires highly calibrated mechanical parts is not very efficient. Not because of the yield of the technique, but rather because of the manutention needed. An electrical drive requires much less daily work and offers similar (or even better) benefits.
It is not actually so. Flywheels with magnetic bearings enclosed in a high vacuum casing can maintain about 97% mechanical efficiency (as per Wikipedia). In fact, they can potentially replace conventional chemical batteries, since it takes only a few minutes to "charge" a flywheel, i.e. to spin it up to its full rotational speed. And the rotational speed limits depend on the strength of the flywheel material...
Just in case, an electrical motor is also made up of high-precision mechanical parts (rotational speeds are not much different)