I think they can work, if everyone involved is truly happy with the concept. I wouldn't want to be in one myself, not at this stage in my life anyway. In my 20s I was very commitment-phobic and had some relationships that were de facto open ones I guess.
They may for a time but.... because having that one person in your life that you love and trust and that only you get to love and trust, gives you the security and confidence to have a happy relationship.
People really don't do ''open'' well.They may for a short time, but human nature doesn't do it well.
For those who have for a good few years, they know the cost.
There is one!
They just won't say....wanting to prove it works, when all the time it has many holes in it.
But...sadly they deceive not just those around them.
At this stage in my life it's not for me. In the past I probably would have been very happy to be in an open relationship, and while I'd be rather happy to have a little more sex in my current one I don't think I could do the open relationship thing.
I think I'm a monogamous creature by nature. I think humans are by and large monogamous, but of course there are some exceptions. I'm always wary when I hear friends have open relationships, I wonder how content both parties are with that, and whether they will both remain on the same page long term. I'm sure it works for some people, I doubt very much it would work for me though.