Open Relationships/Polyamory can work. I know, I've done then.

They take effort, and communication, and boundaries, and respect.. and most importantly of all, emotional maturity..Its something you generally have to have mutually in mind from the very beginning of the relationship.

Most of the time when they crash and burn in catastrophe, is when a monogamous relationship tries to go Open/Poly.. because, at least in my experience, Mono trying to go Open/Poly is almost always because one partner pushes for it, because that one partner has found someone they're gonna fuck/try desperately to fuck regardless, but they at least want to try and engineer permission for it (either before or after the fact). and that shits always doomed to failure.

Not to say a Mono can't successfully convert to Open/Poly, or that there arent Open/Poly that fail even if its Open/poly from the start.. Theres always gonna be exceptions and outliers.

But the biggest factor in the success/failure of any relationship, whether it be Mono, Poly, or Open.. is mutual respect, open communication, healthy and respected boundaries, and effort.