It is very true that there is not a lot of data or statistics out there regarding this. I remember reading an interview a few years back with relationship expert whose career is about working through couples' hardships. He/She said something interesting - that marriages that are changing to be open have a super high percentage of breaking down due to the inherent nature of consensual non-monogamy - the vast majority experiencing issues or pain around some form of jealousy, for example. Granted, that can change, worked out, etc. (And it is important to point out that we're not talking about relationships where that dynamic is there from the start.)
In my opinion, if we're talking about the superiority of one relationship types, there is not one that is better than the other. Whatever works for people, if it works for both, that is way more than ok. Just because there is no way in hell I would be willing to take part in any kind of non-monogamy, ethical or otherwise, does not mean I look down on those who are in open relationships. I honestly think that those who are in the lifestyle would not necessarily have a higher level of divorces, but I am specifically looking at the leap, the change of lifestyle, that's where it can break down more easily, I think. It's not just about one person doing a thing for themselves, you go into it together, two heads, two hearts, and there's no way to know for sure how the other person feels, thinks. Communication is key, but there are so many things that can be interpreted or understood differently, even retrospectively, unexpected emotions, realisations - what if one of them suddenly realises that it is not for them as the deed was done? You can't reverse the actions any more. Someone realising they are unable to accept their partner being touched by others after it already happened will think about it for ever, he/she won't just forget it - as I said, this is not a flavour of ice cream you tried and disliked.
What I am suggesting is that such a change in lifestyle risks so much for not a lot of reward. In OP's specific case, it's not suggested that it was ever part of their interests or kinks, it was coined by a friend. A very weak base to build on, in my honest opinion. Risking an otherwise working marriage for something "we could try" a friend suggested. You said it yourself, you need a strong structure, agreement and communication.
Speaking of biases, if you read back what you said, you know you are doing the opposite on your side of representation, right? How you haven't met anyone who tried and separated, but that is because your circle of successful non-monogamous relationships are amazing communicators and are in love. Please, like that's an exclusive trait exercised by ethically non-monogamous people. I know, you will say "I never claimed that", but just read what you said, there is a very clear biased description to the group you are part of. I will openly say that I am biased towards what works for us, because I find it more special that it is something that we share, just the two of us, a special occasion where we leave everything out, especially other people, vanilla as it may sound. I don't know, we are very much in love still, have very healthy and open communication, and our intimacy did not go boring where we needed to include other people to not be vanilla any more - The fact that we didn't need "outside help" makes it more special to me. We don't love each other any less just because we don't want to share each other with other people, our communication is open and very healthy just because it is not rooted in non-monogamy. To each their own, that much we agree on.
As there's not a lot of data out there to rely on, researches, etc, I guess we all look at what we know first-hand, the people around us: For you, it's the super special, kinky circle of champion communicators, for me it's, well...just people. Average, everyday people. (This should make you think, by the way, your circle is a specific slice of people.. It's like saying, well, all the people I go to swinger clubs with like swinging - well, gee, who could've guessed that.) Yes, to answer your question - I know a lot of monogamous couples who broke up, of course, we all do. And I know a small number (counted on one hand) of non-monogamous couples who are still together - I should not count 1 of them, because ironically, they were partners of other people, and met in a club, where they fell in love with each other, and left their original partners. Nice little segway to a potential risk, I might add. And there are the people who went into it, and it broke their then-relationships: That number is way higher than the first group. Vast majority of these people regret trying it. I am realistic, I am not saying they would have for sure stayed together if they never tried swinging, but that was the reason of the breakups. I don't know where to place the two couples I know who absolutely regret trying it, but stayed together - they have one thing in common: this often comes up during arguments they have because they completely misjudged their own comfort levels and the idea of being kinky blinded them to the reality of what it meant - now that's a great reward for the adventure they took if I ever saw one. And that's what I mean, there are just way too many risks involved. Of course, if both people are on the exact same page, it works for them.