it is the most efficient in terms of energy transfer. I think it is lacking in terms of actual storage capacity. the upfront costs are also extreme.
ideally, every green energy farm would have a gravity well storage system nearby (like pumped storage) for the purposes of keeping discharge steady. Energy would be transferred into and out of this system pretty quickly, and stored in a more cost/size efficient storage option that could act as the true "battery" on dark days / longer periods of energy production downtime
A good example is the large scale hydropower project in Switzerland where they added 900MW capacity to the grid for storage. They do have a good geography for that.
I wonder how difficult this would be to implement for consumers/smaller deployments. Could someone dig an underground pool sized hole for a tank of hot water that can be converted back to electricity?
For some applications you can store heat. For instance you can have an electric heater with a lot of bricks inside an insulator box in your house and move heat out of it convectively with a fan.
For static applications, wouldn't we be better off with batteries based on Sodium chemistry? Less efficient, but removing the geopolitics from one of the main components feels like a pretty big compensating factor.
Don't forget flow batteries using iron, vanadium and other chemistries, as well as various sorts of molten-salt batteries. For fixed installations we could put up with many inconvenient attributes if it gets the cost down.