Not having to build and maintain a dam and find an appropriate area to flood is a pretty large upside. Big thing in the US is that we already have a lot of dammed lakes in mountains in some places so we're really talking about making the water level more variable and adding some infrastructure instead of building net new lakes.

Pumped hydro consumes much less water than evaporative cooling of a thermal power plant with the same energy throughput. Pumped hydro doesn't have to be on an existing watercourse, so it doesn't cause the environmental issues that come along with that.