No Netflix for me, and no luck on a quick search of torrents, but I'll keep my eye out for that show/series.

Many metrics to compare. Ecologically, that system sounds great for static locations with enough of an elevation gradient and reservoir areas to make it work. On the other hand it seems like the ecological damage done by constructing batteries, factories, and disposing of them is likely quite small compared to many other alternatives, particularly fossil fuels (which also have long-term scarcity concerns on top of plenty of other issues).

A major advantage of battery tech over hydro storage would be mobility. If the thing consuming energy doesn't sit in one place, hydro storage won't work. Another somewhat less significant advantage is the ability to install anywhere -- a battery farm recharged by mains and/or a solar/wind farm could be installed in places where hydro storage couldn't. And for one more item in favor of batteries, I'd wager that the land area footprint required for batteries is much smaller per kWH stored, although that might be wrong for extremely large reservoirs (ie. a hydroelectric dam, pretty much). But by the time you're getting to that large scale, the location requirements and ecological disruption are also much more extreme.

Anyway, I don't mean to pooh-pooh the idea of hydro storage -- it really does seem like a very good and ingenious idea where it would be applicable. But there's certainly room for improved battery tech, too. I don't think that we're going to get fully or even significantly weaned off of fossil fuels quite as fast as the video would have us hope for, either. Fossil fuels were the primary tool in our toolbox for a LONG time. And as the saying goes, since all we've had is that "hammer", we've started to think of everything as a nail.