Munkers are effective pending your foes do not know how to counter them, or are a bunch of charging idiots. They can also be used in offensive manners, not just defensive if you can approach and lay out mines ahead with lesmok, forcing foes to either reverse or commit to attacking. Gunners are assets on munkers because of this.
Munkers don't have to be 5 mines, base munker build utilizes 2 mines. If a Junker is able to utilize 2+ mines on two angles of attack, then I'd classify them as a munker because the design concept of the build is to utilize mines effectively in many roles. When you give them the ability to attack more than one angle then you are working mines into the overall strategy of the vessel. When mines aren't neglected or relegated to 1 role only, then you have a munker.
Overall the hardest ship for a munker to down is surprisingly, other junkers. Simple reason is this, the junker moves so slow. Speed of opposing ships actually helps mines be more effective. When the impact hits, they'll tend to be thrown off or into other mines. Junkers are flying turds and can stop their movement rather effectively. You can get major impacts which toss a junker about but a good pilot won't drive his ship into them. I've done Munker matches with all mines before and when mobulas or any other ships was involved, they'd be killed fast. But when its all Munkers, unless you've got a 5 pack going off they're just tough to bring down.