There are problems equalizing any series string, no matter what the chemistry. That's why better lithium tool batteries have a battery management chip on each cell.
Nickel-iron has a very low cell potential (1.2v), which means you have to string more in series to get the same voltage as with lead acid (2.05v).
Nickel-iron also uses caustic lye as an electrolyte, which means you must keep the electrolyte from contact with the air. The CO2 in air reacts with water and the OH- ions in the electrolyte and neutralizes it. It also forms carbonate scale. Replacement water is more frequent than lead-acid and must be high purity water with no dissolved minerals.
Edison was a bright guy, but lead-acid is easier to deal with. That's why lead-acid is universally used for backup power.
There are some sodium chloride-nickel alumina batteries now available for backup power that are supposed to be better than lead-acid, but I don't see them sold except to telecoms and the military. Because electrons can't move across the alumina barrier, they have extremely low rates of self-discharge, and are supposed to last >3,000 cycles at 80% DOD.
