There's some quite interesting emergent behaviour that came up with Roaming Mine, that makes it potentially a very powerful ability. Its movement algorithm is as follows: sort adjacent vertices into three lists; enemy, neutral and friendly; then select a random element of the first of these lists that is non-empty, and move there. Simple enough - it'll basically move about at random, but it would rather go towards enemies (and damage them), and it might as well steer away from friendly territory - it's more likely to find an enemy elsewhere.
But what happens when there's a hole in the middle of your territory; a neutral vertex (or a few) surrounded by friendly vertices? The Roaming Mine will always prefer to move into the neutral area over your vertices, so it'll get caught, never finding its way to an enemy. So there's a risk that they'll never do anything, staying stuck in this hole forever. But the cool part is: this is actually an advantage. It allows you to focus your attacks to maximise effect. When a solitary Mine trundles off and does a small amount of damage, it might end up having no effect whatsoever, if perhaps a Repair Robot fixes things up before another attack is made in the same area. But if you save several of them up, trapping them all in a hole, and release them all at once (perhaps by capturing the neutral vertex they're stuck on), then they'll burst forth like a swarm of angry bees and do all their damage in one go. Much more effective!
I liked this strategy so much that I made the AI check if a gap in their territory had a Roaming Mine in it, and if so not fill it up, so that when I'm playing with AI allies they won't mess up my plans. Useful.
How can it be countered? A well-placed Pulse Wave could take out the whole lot in one go. Charging in and releasing them yourself before too many are saved up could help - especially if you soak up some of the damage with Protection or Fortification. And you could always try doing the same thing yourself - hoarding up your own arsenal ensuring mutual destruction.