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
I know that, that's why I said not because of the yield. The energetic yield is probably better than chemical batteries. The technique is very interesting and can easily work.
But you still have to do regular verifications on this system (probably more than with batteries, because if something goes wrong you would see a 2'000kg disk rolling at more than 100km/h in the middle of town crushing everything in its way).