I agree with your entire comment, but since I know people who love poly despite finding it confusing or difficult at first I can't quite compare it to Marmite. A decent amount of people who don't take to it naturally at first still find it worthwhile once they adjust. If finding it hard at first were a perfect indicator that it will always be unpleasant, I would say those people should let it go. But I haven't seen it to be a completely reliable indicator.

An example would be my girlfriend. She's been happily poly for over a decade, but started off pretty uncomfortable and jealous. That didn't last long, however, and she couldn't imagine switching to monogamy.

Now, I definitely am not one of those people who goes, "If you were enlightened/secure/loving/whatever, you would be poly." It's more like, "If you want to be poly, it's a decent idea to try and work through whatever is bothering you about it." There's no reason to want Marmite even if you dislike it at first, but there are compelling reasons people might want poly even if they find it a bit challenging at first. OP, for instance, states he had some difficulties to work through, but he sounds like he's glad about that.

On the other hand, I do know a lot of supposedly poly folks who actually hate their partner having other partners, but resentfully "accept" it since there aren't many people who will be cool with "I can be with others but you aren't allowed." Those people annoy me.

I guess each individual has to figure out for themselves whether initial difficulty is just a transition, akin to many other relationship growing pains people get through productively, or if poly is actually incompatible with their happiness.