Hmmm...... I would imagine that it really depends on the people involved. A broken relationship is a broken relationship, regardless of whether it's polyamorous or not. That being said, it depends on whether or not the involved parties are strong enough to A). Try to fix the relationship, or B). Strong enough to admit that it's not working, no matter how much admitting it hurts, and try to move on.
If they're too weak to either end things completely or address their issues, then yeah, I suppose you could end up with a healthy relationship and a messed-up, broken mess of one. But I really think that the same thing happens just as often in monogamous arrangements, as far as people clinging to broken relationships because they lack the inner strength to fix things or end things.
This is my family. It may be little, and broken, but it's still good. Yeah, still good.