I think you're missing a crucial aspect of polyamory that is NOT the same as monogamy, which is that when you add additional people to your relationship, you are creating new relationship agreements not only with your original partner but with the new partner/s. And the more people you add, the more likely it is that the relationship agreements will not align perfectly between all people in the relationship. What is a priority between person A and B may not be a priority for person C and B, or C and A.
And when these priorities conflict, and it's not possible to keep everyone happy - the likelihood for conflict among people in the relationship increase, and the likelihood for resentment increase. And this cannot always be solved by "good communication".
Let me give an example of a V shaped Polyamorous relationship between Person A and B, and person B and C.
Person A and B want to have a child together. Person C does not want to be a parent. Before anyone gets pregnant, Person A, B and C spend roughly equal amounts of time together and have a roughly equal amount of energy spent on each other.
Person A then gets pregnant with Person B's child.
This will mean that either person C or person A will have to make a sacrifice they don't want to make, regarding Person B's time. Person A says that Person B should spend more time with them and focus on being a parent to their child. If they do not, they will feel like Person B is shirking their parental responsibilities and also abandoning Person A in their time of need.
Person C feels that Person B is spending less and less time with them, and feels abandoned. They do not want to spend all their time looking after the baby as it's not their baby and do not want to be a parent. However any time person B takes to spend with them, is time taken away from the parental responsibilities with Person A. Person A feels like they do more parenting work because they are in a V shape relationship, and they don't have a lover that they can just go spend time with alone without the baby. But Person B does. And so resentment builds in both Person A and Person C. Person B feels caught in the middle, trying to make things work with both relationships, but ultimately cannot. In most cases, Person B will become monogamous with Person A because that's the only way that they can co-parent fairly.
In this case, neither person A, B or C acted unethically. They tried their best to make things work. But they couldn't, because of the competing priorities in relationships.
This is not inevitable in polyamory but it's an example of how when more than 2 people are in a romantic relationship, competing priorities will not always align and will not always be able to be resolved in a way that's satisfactory to all parties involved.
I gave this example because it was something that I was thinking about when I was in a V shaped polyamorous relationship in the past with my partner (who was also with another woman). While none of us had kids, it could be balanced. But if either one of us women got pregnant with our partner, I believe it would have been a dealbreaker for the other partner to leave the relationship. I could not tolerate being sidelined while my partner focused all his energy on being a father to the metamour's child. But if he didn't do that, it wouldn't be fair to her or the child. So I would have to break off the relationship. Likewise if I got pregnant, and my partner did not spend more time with me, focusing on being a father to our child, I would be pissed and feel like he was shirking his responsibilities as a parent. So he would likely have to break things off with the metamour, or she would have to be OK with him reducing the amount of time he spent with her - or one of us would have to be OK to become a coparent to a child that wasn't biologically ours. Which I don't think either of us wanted.
The more people are involved in a relationship, the more complex the situation becomes, and the more complex it is, the more chances there are for things to go wrong. It's not inevitable for things to go wrong. But to imply that there is NO DIFFERENCE between monogamy and polyamory is just wrong. Polyamory is inherently more complex.
