My I-think-conservative WAG for the cost and land-area-use differences between England and Switzerland, for this kind of storage, would be about 50x and 5-10x, respectively, for similar amounts of stored energy. Both elevation and grade of the surrounding land matter a lot for how much energy you can reclaim per stored liter, and having two or three sides of your reservoir provided by nature in the form of steep-walled valleys has to provide serious cost savings.

Like of course any water you push up a hill is gonna gain potential energy, but I think a reasonable reading of "works" in that post was "works efficiently enough that it's actually worth doing", not "is technically possible".

Your "wild-ass guesses" for costs are worth as much as you say. Grade, less: it affects mainly how long the penstock must be, but pipe is cheap.

An earthen berm is a very cheap construction project. Classically those were achieved with hand labor worldwide, for millennia, eventually augmented with wheelbarrows starting in China, and oxcarts. They cost less today.

Nature providing one, two, three, or four walls (steepness optional) reduces construction cost, but the turbines and pumps are a substantial part of the total.

Opex is of course extremely low. Bonus points for solar farms floating on the reservoirs. Round-trip efficiency of pumped hydro is near the top of all alternatives, although efficiency is relatively unimportant when the power being stored was free.

Not seeing how the physics can possibly work out for the real costs and land-area-use being too much lower than my guesses to achieve the same total stored energy, but sure, maybe there's some engineering magic I'm not aware of. Could be. In particular, maybe I'm overestimating how much more energy one can efficiently extract from, say, a liter of water dropping 5,000 feet over three miles, versus a liter of water dropping 600 feet over three miles.

Anywhere pumped hydro would cost more than another of many storage alternatives, the other alternative will reliably be chosen. England is, e.g., preparing hydrogen storage in depleted NG cavities. Pumped hydro will be built in the places where it is cheapest, favoring high hills and variable topography.

Costs for many storage alternatives are falling fast, and it is unclear which choices will win. Adding pumps to existing dams has lately been cheapest, by far. Future people will choose according to local conditions without asking us.

Right, I get that there are alternatives that would be used instead, what I was surprised at was characterizing "hydro only works in mountainous areas" as flatly false, if we take "works" to mean "is worth doing", which is a common usage and I think a fair reading of it, and since it seemed to me like there would be large differences in the economics of it, depending on local geography.

Low hills, where the hilltops are not otherwise in use, may suffice. Some places have very deep gullies or underground cavities that would serve in place of or augment hills.

The point is that whether a storage medium is practical in an area depends more on the landscape of choices than on the physical topography. Even Nebraska might have deep cavities that would serve in place of hills. But the extrema are extreme.