Flywheels that don't become fly-apart wheels is one problem.

Containment of fly-apart wheels another. Buried concrete pipes are usually what you're looking at. You need mass to contain these things.

The per-kg energy densities just aren't all that high. If you want reactive load to soak up very short-term demand / supply fluctuations, you can get that. And you've got crazy charge-discharge cycles.

Bearings are mostly solvable with magnets, as I understand. But you still lose a given percentage of storage over short time -- ~1% per hour or so, within a few powers of 2. Enough that long-term storage isn't viable.

And the thing that really screws you over is that the Earth moves. Geometric precession of your flywheel is something you've got to deal with.

There's a place, but I don't think it's large-scale, long-term grid storage.