> Closed loop (off-river) pumped hydro storage has the smallest carbon emissions per unit of storage of all candidates for large-scale energy storage

Pumped-storage is a rather special approach as it feels almost like an extension of the natural water cycle.

Reducing the total footprint of the energy system (including all materials required for construction and ongoing maintenance) is an important part of the sustainability puzzle.

Ofcourse once you scale things to the gargantuan energy needs of modern civilization all sorts of minor "side-effects" may become important.