Batteries have the very large disadvantage that cost is per Wh stored. Flow batteries can reduce this, as capacity becomes a matter of tankage and electrolyte.

The sweet spot for storage is near-zero opex, very low per-Wh stored capex, and cheap capex per-W in and, particularly, out. Thus, weight for watt-hours, and simple motor / generator for watts in and out.

Synthetic fuel has the appeal that the stored energy is readily tanked and transported, and very saleable; enough so to overcome low round-trip efficiency, expensive electrolysers for W-in, and complicated combined-cycle turbines for W-out.