My opinion is strictly anecdotal; I’m not a professional, I can only speak to what I’ve personally seen, and that may or may not be representative.

OTOH, if sex and relationship counselors are saying that the overwhelming majority of people are doing multiamory badly, then their opinions have a lot more weight. Are they necessarily correct? No, of course not, any more than the opinion of any one doctor could be full of shit (see also: any doctor that thinks trans-ideology is a woke-mind virus, or whatever they’re saying now). But it has a lot more weight than opinions of non-professionals.