Many of us, me included are speaking from limited experience and non-direct experience too. I only know about poly relationships from friends who have been in them or at least experienced them growing up. I only have direct experience of 1 open relationship.
It's really not our fault for only having limited experience positive or negative, we all base our opinions on what little we know or have seen. In my case I've only experience or knowledge of coercion and abuse. I accept it's not everyones lived experience that their relationships are like that. Coercion and abuse is common in all types of relationships, most of us have experienced a terrible one at some point. I do become a little concerned that with the addition of more people, there is more scope for abuse. Not that poly is especially susceptable to abuse, I'm just thinking about numbers and opportunities for abusive people to misuse this kind of relationship, I'm happy to be wrong if anyone has any research on that...
From my 1 experience I know I am unlikely to choose to enter an open relationship again (never say never) but I do accept that is my choice to make for me only and not for me to force that choice on others.
If I was asked to offer advice to anyone thinking about poly and open relationships, I'd suggest that they gain as much information on them as they can, talk to people living it, get to know the people you are entering into this relationship with, consider the impact on yourself and others in the relationship, and go into it with your eyes open. I thought I'd done enough legwork and talking before my experience, I thought I knew what to expect for myself but I obviously didn't consider all the points, like how negatively it would impact my ex who was the one who really wanted this type of relationship in the first place and yes he is a man, stereotypes and tropes are there for a reason.