--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.
P.S.: Fire bombers do the same job better and faster.
In case this is not a troll, let me say that I am always delighted to party with skilled gunners. The good ones know how not to disrupt their teammates (for example, causing mecha knights to shield at bad times or obscuring everything with Polaris explosions). The good ones pick off healers where swordsmen can't reach them easily. The good ones take advice when it's good, although I have trouble understanding why this comes up so often for you.
Edit: As a swordsman, my most troublesome scenario is a field of turrets protected by spikes. In this situation, I shield while a gunner fires constantly from behind my shield. This is fun and effective. Does Ash of Agni do as well, in this situation?