Type XXI submarines and historical reputation

What might make a Type XXI submarine qualify as an ultra-rare event ship in Azur Lane?

The developer can assign ultra-rare status if the Type XXI will attract players; the discussion does not treat the game's rank as a historical judgment.

The discussion does not argue for an ultra-rare rank from the Type XXI's historical performance. The decision belongs to a game developer trying to attract players to a Chinese anniversary event. If a Type XXI character can draw buyers, the game's designers can assign whatever rarity and story treatment supports that goal. Its in-game rank should therefore not be read as a comparative judgment about the submarine's wartime capability.