It’s going to depend on the details. In a power substation space and weight are fairly flexible. In a vehicle they matter quite a bit. If you can build a flow battery that rivals solid state batteries for weight and volume, you’ve made an improvement because the fluid is swappable in ways that batteries are not. Batteries meant to move will move in a crash. Fixed batteries tend to stay put but complicate road trips.

In a UPS the tanks and valves and pumps are new points of failure, as the other responder states. In a car they are many times more complicated than electric vehicles, yes, but still less complicated than ICE vehicles by far. Your heater has more moving parts. Hell, the emissions control gear is probably more complex than the entire drivetrain of the electric vehicle.