I know of two couples that dabble in it to some extent. One as far as I know is unicorn-hunting, because their rules for it suggest a 3rd member genuinely capturing someone’s heart would lead to relationship implosion of epic proportions, and I suspect that couple isn’t mature or stable enough to be doing what they’re doing without leaving people open for hurt. Not that I have any say in it, lol. But I feel sorry for any thirds that interact with them thinking there’s even a chance of them being an equal partner.
The other couple has much better communication skills, and claim they’re poly, but as far as I can tell from the outside “poly” has happened as an attempt to save the marriage. Maybe they’ll make it work, but I’ve watched them make some dumb mistakes, and the wife has jealous behaviors when women interact with the husband and a history of bending to his needs before her own so I think even if she says they’re poly she might have talked herself into it as a way to attend to him.
I think healthy poly is possible–but it requires extremely mature individuals with exceptional communication skills, and that’s rare even in monogamous couples.