--1) Party UNloyalty. They generally get excited from the range and don't even pay attention to the very noticeable damage decrease, which results in them poking their targets, if they even hit. (Auto-target mildly, yet negligibly covers this.)
--2) Close-mindedness. They won't take advices from other knights, even if they're very relevant and correct.
--3) 1v1 yes, 1v100 no. Gunners are never good against waves of mobs. No exception possible.
All in all, if you'd ask me, gunners should simply constantly go solo, or wisen up. If they do, MAYBE they can shine.