> The Earth also rotates on its axis. And with the speeds and energies with which grid storage flywheels operate, that's a significant engineering concern.

You can align the angle of the flywheel to match the angle of the earth at your latitude, which helps with that. However if you do that the flywheel is titled, so the bearings are uneven, and then you need some shielding above the flywheel - you can't just have the earth be the walls.

But it means that the bearings experience much less force, so it can be worth it.