I should have actually expanded on that a bit because I was continuing the conversation about the instances where one of the existing couple gets hurt opposed to the unicorn. If I was intending to find a poly person and do a cowgirl move, I'd target unicorn hunters (opposed to a poly couple open to a triad) because they already have the instability necessary for me to worm my way in. I believe this makes up one group of couples who are harmed by their unicorn hunt.
Also, with typical unicorn hunting couples, the chances that one of the dyad aren't really on board with the poly thing and consequently want to pull out are high. Unfortunately (for them) the other person doesn't always want to pull out and either leaves them or cheats after saying they'd break it off. And we have another group of couples who end up the aggrieved party when the triad implodes.