Leviathan's Breath is a joke. Outside seasonal content where you need an unstoppable weapon it's useless. Damage is low, charge time is high, ammo is few an scarce and it takes an exotic slot. It's almost as bad as Xenophage and Oathkeeper doesn't do jack to help it. And I haven't seen anyone use any other exotic bows in the game. Subtle Calamity with a good roll or Hush are the go to bows for PvP if you were so crazy as to play bows in PvP in the first place. As for PvE... Well, other weapon types exist.