These are good. The strength is also their problem. While lead or lithium batteries plate/unplate their respective metals, the flow battery changes the state of a charged ion in solution - the state change either stores/discharges the energy. That is the problem, solution limits. They consist of 2 tanks, one for the charged and the other discharged states. For fixed installs, the tanks can be the size of swimming pools = huge. The rate limiter is the charge exchange stack. A smaller stack = lower continuous power, and larger = greater.
More details/refences here.
https://en.wikipedia.org/wiki/Vanadium_redox_battery.
As you can see it has a lower round trip efficiency - depends on how hard you force the exchange stack. On the + side, they can sit in the charged state for decades.