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Pumping water comes with host of fluid-related inefficiencies. And you need pressure vessels. The big advantage of the OP is that there are no potential leak points. Worst case scenario, cables break and the rock drops.

Yeah, this is another promising one. The other disadvantage with that design is that the weight of the water is actually working against them. So if their rock/concrete is about 2.5x heavier than water, the weight of water is subtracted and you only get 1.5x the weight of water. Also their height is limited by the height of their piston. So I'm not yet convinced that their solution is better than just using underground pumped hydro.

It may be so. But an underground pump hydro is just boring.

Unfortunately, Heidl Energy is no more: "Heindl Energy GmbH ran out of cash in 2020 and filed for insolvency. The patents and the engineering work has been sold to an interim investor, New Energy Let´s Go, who is now looking for a new strategic investor willing to continue the development with the goal of constructing a pilot project to prrof the concept."

from: https://heindl-energy.com/about-us/