Flywheel energy storage sounds more explosive than battery energy storage. Batteries burn but a giant near supersonic (at the rim, even in the near vacuum I'd assume these would need) spinning mass? It's great when everything is running perfect until it seizes. I'm sure the engineering issues can be solved but it sure doesn't seem easy.
To be fair, supersonic in near vacuum is pretty slow.
I think risk on both technologies probably hand-waves out to equivalent. In a farther future surplus energy situation where inefficiencies can be absorbed, one could imagine the lifecycle and safety of mechanical storage being pretty attractive. (Common materials, buried devices, etc.)