For the more technical minded, I have uploaded half a dozen or so papers on pumped hydro which discuss its benefits, costs and profitability in deregulated electricity markets:
Even if you don't understand, or don't want to read the papers, at the very least it might pay to read the abstracts and conclusions. I read these papers as I was doing my DD and frankly, came away with the view that pumped hydro is anything but a sure bet. I forget which papers I was reading, but two findings stood out: firstly, any cost blowouts during construction will massively curtail future profitability (unsurprising, but this is only exacerbated by the second point) and secondly, year-to-year EBITDA is highly variable. We're talking variability +/- 100% YoY here.
That GNX has been able to add a PV plant to their portfolio with a market-leading capacity factor has significantly de-risked the company (at this price, at least) IMO.