My monogamous marriage ended after 17 years. Did it not work? Or did its longevity, despite its end, mean that it did work?
My current relationship (a Poly V - I'm Mono and one of the arms (legs?)) is going into its third year with everyone. It's work, but it's respectful and good.
I don't know how to measure whether or not a relationship works, but my relationship with my partner is working. It has hurdles, but they're not insurmountable, and yes, our relationship with each other is wonderful. I think that's the key to any successful relationship, regardless of configuration.
Summary: Don't worry too much about the configuration of the relationship as a measure of success. Look at the people in it.