Pumped hydroelectric has good capacity, but doesn't scale well. The right geography is the main limiting factor, accessibility is a second. The places with good hydroelectric potential tend to be very remote, in the mountains. It's extremely hard to do large earthmoving and concrete construction in remote places. The places that have simultaneously the right topology and relatively easy accessibility are limited.

The alternative is essentially big water towers, and that has serious construction challenges not solved yet.

Pumped thermal storage has no geographic limits. The idea is: compress argon, transfer the heat to thermal store, expand the argon back to the starting pressure to recover some work, then transfer the "cold" to another thermal store. For long term storage, use very cheap materials, like rocks, for the thermal stores. To discharge, reverse the flow, so argon is cooled, then compressed, then heated, then expanded. All this can be done with existing technologies and with a round trip efficiency of perhaps 60%.