Open relationships, like any relationship model, require a lot of communication and mutual understanding in order to thrive. When you add the potential for more partners, combined with less time for date night, it can quickly create tension and trigger jealousy if both partners aren't truly on board. And because mainstream culture is still geared towards monogamy, there aren't as many examples of what it looks like to have a healthy open relationship.
That said, one of the most common failing tropes of an open relationship is when the partner who opens the relationship freaks out when their partner ends up taking advantage of the very relationship model they pushed for.
In these cases, it's often made evident that the person suggesting an open relationship only wanted that freedom for themselves, and don't have the amount of trust or self-confidence necessary to handle their partner stepping out.