This looks to be not much different than pumped hydro, which is a good bit similar (in the sense that it's also a 'gravity battery' (potential energy stored as a gravity differential).

As far as battery technology requiring rare elements, iron flow batteries seem to require just iron and salt water. Like all flow batteries, the 'expensive bit' is where electrical energy is converted to/from chemical energy. Need more instantaneous output, add more of that bit. (similar to adding larger hydro generation/pumping gear with pumped hydro). But if you need more storage capacity, just add more electrolyte and tankage (similar to building a bigger storage reservoir with pumped hydro).

Compared to Li-ion tech, it's highly inefficient in both power storage per weight, and power storage per cubic area.

It's not clear to me how IF batteries compare to Li-Ion, in the sense of instantaneous startup. Li-Ion's fast enough that there's no need to have standby generation spooled up - but even if Fe-flow is relatively slow, a limited amount of 'bridge' Li-Ion would fix that.

As it is, utilities in my area tend to keep natural gas fired turbines spooled up, just to have the instantaneous capacity when needed. I've been puzzling over why they aren't using 'bridge' Li-Ion storage. It's either cost, corporate inertia (something utilities are famous for) or I've really misunderstood something.

Any utility nerds care to fill me in? I'm fine with equations if you want to explain that way.

also

>>will last far longer than a massive lithium-ion mega battery!

To the extent that there are moving parts, I'd hesitate to make predictions. Yes, Li-Ion batteries have a lifespan, but so does anything with cables, pulleys and bearings.