Well, that might be part of the problem, but the main problem is the toxic 'norms' and social scripts around relationships whether they are monogamous, polyamorous, relationship anarchist (though less with this one because breaking down social scripts and removing toxic norms is the whole point of it), or otherwise.
That's the real cause so many problems happen in relationships, not the type of relationship but the assumptions and expectations so many have and that they refuse to take no for an answer, don't respect boundaries and limits, or just refuse to be honest with themselves or others, and refuse to get therapy etc too.