I actually have some experience with this. One of the best relationships I was ever in was a fourway relationship. It wasn’t long term, of course, but it was a blast while it lasted.
Also, my current girlfriend was married when I met her, and for a while she stayed with her husband while seeing me. She wanted to be completely sure about herself before making any permanent changes to her family.
But neither of those were true poly relationships. The fourway relationship was just something we fell into, and my girlfriend always knew she’d have to choose between me or her husband. Firstly, she was a latent homosexual who was always going to have to leave him eventually, and secondly, the idea of having both of us just didn’t appeal to her. Or me. Or even him. It was something we discussed, all three of us, but none of us actually wanted it.
So, that’s what I think. Poly relationships can be great, and even practical in some cases, but it all depends on the circumstances and the people involved.
That said, I’m not sure how long term a poly relationship could be. True love may not even exist, but if it does, I doubt it could exist between more than two people.