The leakage used to be considerably higher like 25%/month. They offgas hydrogen, so you need a well ventilated area, and they are subject to freezing. I thought the roundtrip efficiency was closer to 60%
They are tanks though. They were used a lot in military and telephone operations because they would last a long time, and they can be abused and fully discharged without harm. There are still some functioning batteries that are like 100 years old now. However, they have been replaced with other battery chemistries now.
i don’t know how much nickel is, but the patent ran out of them a long time ago. You can literally make them.