It is true that nickel iron batteries must be topped off with distilled water at regular intervals. When NiFes are used as recommended in our Iron Sun box, this interval is once per month. Other websites claim that NiFes need to be watered weekly. We have never found this to be true, although if they are charged with a oversized PV panel and no charge controller, they will boil off electrolyte more quickly.
Another criticism of NiFes is that they off-gas flammable hydrogen gas. This is also related to how hard the batteries are being charged, and how big the system is. For a very large system, cycled hard in an air sealed room, the off-gassing could in theory be a concern. Iron Sun boxes are small, and in a room with reasonable ventilation, they do not represent a fire hazard from off-gassing hydrogen.