No limits fallacy - it just means that as long as Nagakami Armor doesn't get an explanation about its mechanism, its hax will only be used on people who are below or equal to the strongest person it was used on.
It was scaling with the dragon kings
It wasn't even scaleable because Zeref wasn't serious at all and Zeref isn't scaleable to the dragon kings either
Silver showed that he was more proficient on it than Gray by casually freezing Sun Village while Gray barely freezed a tornado
1) Sema has kinetic energy that backs up its insane DC. OS wasn't taken down because Jellal wanted to recruit them.
2) Jura's facial expressions indicate that it's at least as strong as Roaring Thunder, which is arguably Laxus' strongest attack