Green Energy

Don't get me wrong, I love me a good pumped hydro solution, but they do come with a couple of disadvantages:

a) as others pointed out, you need a somewhat steep gradient between two places. Preferably for a bit of distance so as to increase efficiency by putting more turbines in. Their need for a gradient limits their use in flat locations such as the entire US Midwest, for example.

b) comparatively expensive and longer construction process than other storage forms.

c) usually you have to build the upper reservoir. That's an environmental harm. IMHO not that big if a concern, but it's there.

d) if you don't build out a reservoir, but use an existing lake, you risk contaminating it with algae / cyanobacteria and wreaking havoc on fish and other wildlife.