The people involved are almost always fat, autistic, nonathletic, and have copy-paste nerd interests (Star Wars, LotR, computers.)
That's what I've observed, with one specific exception.
I've known three male acquaintances who went poly and two of them were spergy NPC fatties. One got married around age 30 and in less than a year his hambeast wife said "poly or quit" and he went along with it. For a while he would talk about how great it was when she would bring some girl home for a threeway, which I think happened a handful of times. On her end, she quickly obtained a boyfriend with whom she would spend a couple of nights a week and even a lot of holidays. It's utter, utter cuckdom, and I believe he put up with it because she makes the majority of their household income while he's an underemployed doofus. The other NPC fatty is one of the biggest wokesters I've ever met in real life, living off a tugboat for anxiety or some equally bullshit thing, and spending all his time on social media echochambering with other lefties, sucking up as the cis-het-white-dude doormat. Both of these guys are thoroughly unaccomplished, unsuccessful men with no power in their relationships.
Which brings us to the exception. I do know one guy (also a serious wokester) but also a competitive athlete with a very cut body, and a solo homeowner with a good salary. He got into a good career very young and for a couple years, he sucked up the loser life living at home with his dad while making good money so that he could then buy a relatively new house in an affluent suburb. This guy is now fucking
killing it at the poly game. For a while, a married couple moved in and rented a room from him, and he was openly schtupping the wife. They're out of the picture now but he's still renting rooms to a couple of people, and generally fucking at least one of them at any given time. This dude is living the life that the other poly guys wish they were, but it works for him because he is attractive and successful. Different power dynamic, different results.
Like most things, poly is a lot better if you have money. It doesn't work for schlubby guys who think it will change their lives. It just ruins their relationships and puts them in awful, shameful situations.