Flow batteries are the opposite of everything else
I'm surprised that Redflow haven't mentioned that flow batteries (including theirs) are different to all other batteries. The prefer to be empty, not charged. Not so good for a backup application, but OK for solar load shifting. And they need pumps to circulate electrolyte, and those pumps use power.
I've never considered lead-acid charging to be complex. They float charge with a constant voltage source quite nicely, and don't have the thermal runaway problems of more complex chemistry. They might have a low energy density, but how often do you pick up and move a battery bank that is not installed in a car?