There are a lot of challenges in maintaining multiple relationships. One of the trickier bits, it seems to me, is when partners’ needs for intimacy and presence don’t match.
I am not someone who needs regular in-person time to maintain a feeling of intimacy. (If I were, my long-distance relationships would be impossible.) By contrast, one of my local partners needs time every few days to maintain that familiarity.
That could work out fine if your constellation of relationships includes people with different time requirements. But if they all need frequent in-person time to maintain the relationship, you’re going to wind up polysaturated much sooner. And if you are someone who needs frequent time, choosing partners with fewer existing commitments becomes a must.
If someone else needs more time than you do, they can seem greedy. If someone wants to give less time than you think you need, they can feel distant.
Everyone has different abilities to give time and different requirements for having it. And those abilities and requirements can change as other parts of their life change. Managing those different needs within the amount of time available is one of the most challenging parts of polyamory.