I'm not up to speed on flow batteries, but from what little I know, they seem ideal for grid storage since the capacity is limited to the amount of stored electrolyte, which can be collected and recharged at a later time.
No idea how the cost per kWh compares to other chemistries.
I do have a friend who has a PhD in chemistry, with a focus on flow batteries. Haven't talked to him in a few years.
https://www.linkedin.com/pulse/dont-...-alex-bistrika
https://www.researchgate.net/profile/Alex-Bistrika
https://www.aiche.org/community/bio/alex-bistrika