[Bicycle Bob]

Flywheels have to be used in pairs to avoid major gyroscopic effects, and the bearings have to deal with major forces between them. However, bearing failure is not a major problem - there can be two or three backup modes during failure. The usual concern is over disintegration - the NHRA used to mandate armour plate around the flywheels on dragsters. Modern, high-speed flywheels are made of composite fibers, and turn into fluff when they fail. Unfortunately, it is a lot of high-speed rotating fluff that wants to expand as much as any other explosion, and it contains all the energy of the "fuel," without waiting to be mixed with air. The armour is cheaper than when dealing with chunks, but it still comes to about as much material as an air tank to contain the same energy.