The installation process dominates overall effort, so if that's a lot harder then it ends up being a lot more expensive and thus a lot less cost competitive. It is absolutely insane how expensive an underwater oil well costs to tap compared to one on land, for example.
> Depends on the geology of the sea bed. Where I live, the sea bottom is mostly solid rock, it’s possible to attach stuff to the bottom without putting much ballast.
It's likely to be more expensive to attempt to attach to the sea floor than it is to just sink large weights. In fact, I'm not aware of any large structures that are anchored to the sea floor in a way that resists an upward force rather than a downwards one. It might be a wholly unsolved problem.
> The idea’s nice I just worry about disasters.
Falling construction cranes are dangerous because they tend to happen in inhabited areas, adjacent to other buildings. The energy storage cranes, on the other hand, would likely be in the middle of nowhere where land is cheap, so even if they do fall there's nothing to damage beyond the energy storage installation.