Yes indeed, there are guns that are far better for PvE that are legendary. But you're missing the point, Black Hammer [u]is[/u] an exotic thanks to the White Nail perk. Bungie can't just make it an exotic and keep the perk as it is.
If Ice Breaker was nonexistent than yes, BH should be an exotic but that's not how it is.
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Theoretically raid weapons are exotic grade legendaries as they're supposedly hard to acquire (talking VoG and lighthouse, not the CE/PoE fiasco). Black hammer is a raid weapon and it's perk is highly situational
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I am highly aware of that, so what is wrong with having to pick up some special ammo to burn a boss with Black Hammer? Nothing.
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There is nothing wrong with it. But it's unnecessary, it was fine as it was.