No, I've seen a polyamarous relationship last longer than some of my friends' marriages, because the people involved know how to communicate with each other, rather than turning around and ragging on their SO to their friends or parents about fights, or refusing to say what annoying habits are annoying. They talk about it, they figure out solutions, and then they get past it.
Any relationship where communication is put first, partners are equal, and emotions are talked out is going to work better, regardless of orientation or the people involved, monogamous or polyamarous.
None of this is to say monogomy is bad, or can't happen. My mother's been dead eighteen years, nineteen this year, and my father regularly gets pressured to 'date' and 'move on' and I get people asking me why he doesn't take off his wedding ring, or people who accuse me of telling him not to date someone else. My dad is monogamous. He loved my mother, and he said vows. To him, dating someone else would be being unfaithful to her.