A queer male play partner of mine is married to his queer awesome wife. They both have a variety of play partners, have a lovely house, have a live in house boi, and have been together for years. After pegging him one time, I fell asleep in their bed and wandered out into the main part of the house to see his wife coming home. Due to my very small amount of personal experience with poly relationships, I initially panicked. I was genuinely concerned about what her reaction would be. We had only met once prior to this. Instead we all just chatted about how her play date had been, drank some coffee, and hung out.
Any poly relationship I'll be in will be completely open. I don't mean that I have to become best friends with my partner's partners. I just am not okay with a "don't ask, don't tell" policy. I'm still play partners with the male and I've gotten hot pictures of his wife as a part of his pervy/kink friendly Christmas card. The only other experience I have being poly is when I was dating a sissy and one of my best friends. The relationship didn't fail due to it being open. It failed, because I wasn't that attracted to my best friend, and because the sissy had some personal things she needed to deal with. All 3 of us got along just fine. We all had dinner together/socialized for a special evening on my behalf too.
I've spent some time around swingers and... it isn't for me. I've had one night stands in the past, and I've had a relationship that lasted years (not bad for only being 23 I suppose). While my desire for sex has very much changed from what it was, if I feel a need to seek sexual companionship, I would find a friend with benefits situation again. In the past I had 3 fwb during the same duration of time. Safe sex with all of them. One got a girlfriend so we stopped fucking. Another moved. And then I ended up dating someone so I broke things off with the last one. Not that I'm implying there's anything wrong with casual sex, so long as you're doing so safely and honestly.
tl;dr
Poly relationships and swinger relationships certainly can work. Happily and long term, even. Considering the massive amount of divorce rates in the USA and the relatively small percentage in comparison, of people who are in open relationships, I certainly wouldn't blame that as a factor. Just like any relationship, people can fuck things up. That isn't necessarily just because it's a different form of romance vs. one + one monogamy. I don't personally see myself ever having that type of a relationship. There isn't likely going to be any one single person who meets all my needs, and I'm ok with that. Do what makes you happy without being careless of other people and be honest.