Here in Missouri , we've had pumped hydro for 40+ years before having any problems.
Back in 2005 it failed, with no lives lost. It was completely rebuilt holding upwards of 1.5B US Gal of water.
There's a few people on U-tube doing solid mass and water flow posts. Since water's density is well known, math is an easy exercise.
There was a very nice were, a member posted all about his wall crawler Gen he built using Lego blocks. Was cool to read, and it worked.
Highly efficient? Not likely, but he knew that , more of why not when you have the items already available. Also a good exercise in math.
Cheers
Bruce S