Flywheels sound exciting but their energy density is about the same as Li Ion batteries (even when carbon composites are used). There are very few large scale deployments of flywheel energy storage systems in the world despite lots of active interest in them. Safety is also a huge concern since if there is a mechanical failure, a huge amount of kinetic energy will have to be safely disposed or you are going to have shrapnels flying everywhere.
Flywheel systems last considerably longer than batteries, and are generally installed dug into the ground where failures cause no safety issues. Even when not installed underground the vessels fully contain any failure in any case.
In certain failure scenarios I think an energetic flywheel is predictable: Most of the momentum is within the plane of rotation, which is where you can place whatever kind of shielding or catching-material.