It's not comparable. Gasoline is limited in how fast it can release energy, by how much air it can get. So you get a fire but not an explosion.

In contrast a flywheel can (and will) dissipate 100% of its energy at once, leaving you with white-hot pieces of metal if your containment worked, and supersonic pieces of metal flying everywhere if it did not. (Or some combination of the two.)

It's far more dangerous. The amount of potential energy is irrelevant, it's the power (how fast the energy can be released) that matters.