I was a "lifestyler" and I've been involved as a "secondary" with a married couple whom I would also classify as "lifestyler"s.
Long story short: my spouse and I are getting divorced but remaining friends. My spouse's "secondary" is now his "primary" and they remain open. I am no longer anyone's "secondary" and I have no idea the state of the couple I dated. I saw red flags and cut loose. I'm learning how to be on my own now, something I have never done, and plan to continue developing the ideal of openness in my relationships.
Life is messy. Love is a risk, no matter how you practice it. I think you do your best to minimize collateral damage by being as honest with yourself and others as possible; by going slow; and by not getting your heart set on any single outcome. But mostly, by being secure in yourself. Is any of this insight specific to poly, definitely not. It's really just about trying to be the best human being you can be.
I am thankful that poly was there to help me navigate my way out of one way of living and into another. I knew we weren't a perfect couple when we started down this road but in hindsight, I don't think we treated our "secondaries" unfairly. My husband's girlfriend and I got along well from the beginning and never had trouble communicating. My other former metamour and I, well that is another story, but I think we all went into it knowing it was a risk. We went slow. We tried to communicate a lot. We thought we knew what we were doing but some things just can't be planned for. It was a calculated risk. It's definitely possible we got lucky and things could have gone a lot worse. But poly allowed us to renegotiate our relationship in a way that I hadn't known was possible.
"It is no measure of health to be well adjusted to a profoundly sick society."