I have no idea whether there are containment or environmental challenges that make this especially hard.
A Li-Ion battery can charge and discharge in something like an hour. If you double the number of cells in a battery or installation, you can store twice as much energy and you can draw twice as much power. (You don’t have to provision twice as much power circuitry, but you do need to purchase that extra power worth of electrodes in the cells.)
If you’re building something for which a roughly predetermined power to energy ratio (i.e. hours of use at a reasonable rate of discharge for the technology), this is fine. Similarly, if overprovisioning power or energy is not a problem for cost or weight, also fine.
But for grid use, wide differences in the duration of energy storage for different purposes can make sense. And a flow battery can separately provision power and energy. This gives a possible cost benefit to flow batteries.
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.
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.
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
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... ;)
It also have lower energy density so you pumping 600l of new electrolyte on the station might still give you only 200km of range
or maybe a locomotive, which is less sensitive to weight and cargo capacity?
For locomotives ? I'd imagine the simplest solution would be to have "battery wagon" ; get on the station, replace the battery wagon, and be on your merry way while it is recharging in the station waiting for next train to pick it up, at least for the long haul stuff. But capacity density still matters, if flow batteries are being close and cheaper then it makes sense, but if you need to pay slighty more per KWh but get 2x the capactity... thats 2x the range and less maintenance.
Or, you know, just electric rail. It already works fine in many places. I wonder if making some hybrid solution with electric rails also being covered by solar panels (basically so same maintenance crew can manage both) would make sense
So it all depends to what level of density it gets. If it is same capacity per kg but lower power there are still many places it can be used, if it is much lower it stops being sensible for moving stuff. And the price would need to be significantly below the normal batteries for anyone to even bother.
Trains maybe, but surely the money is better spent on overhead wire? Even if tunnels/bridges/etc. can't take it you're better with just the power-buffer battery and no flow battery.
And rail is far cheaper to operate once built per unit capacity than trucking. If you don't inflate it with endless feasibility studies and contracts that get paid regardless of output the initial building costs are about double a 4 lane highway, but it carries a lot more and maintenance is lower.