Surely it is more complicated. It may be able to serve the goal of eliminating fossil fuels from the car while not getting the full simplicity of (not really) solid state batteries we have today.

It would seemingly also allow you to carry around 50 miles of juice when you are running around town and bump up to 400 miles of juice when you are taking a road trip. This really only makes sense if the car is able to recharge the juice.

Maybe a new hybrid model (not gas electric hybrid) could emerge. The base (say 50 mile) capacity is solidish state like we have today and a flow battery range extender could be filled with fluid when the extended range is needed. This would make it so the car wouldn’t need to charge the fluid, if that helps.

I could see a hierarchy of power in a car, or truck/rv.

motor <-> lithium <-> flow battery

the low current from the flow battery could be continuously augmenting and replenishing the lithium ion battery

or even:

supercapacitor <-> lithium <-> flow battery

Being able to "refuel" quickly is a pretty big plus, so in the balance it could still be better to accept some moving parts in exchange for that.

And it's easy to imagine retrofitting a gas station to all of a sudden have charged electrolyte instead of gas, but I'm sure that has all sorts of additional complexities that I know nothing about... ;)

Probably adds two low-pressure low-volume pumps. Which is more, but not a large increase in complexity for a car. The driver's seat probably has more moving parts.