cults often practice polygamy as a few men in power "marry" girls (teenagers or younger), and control the assets. this is a power imbalance that can easily be abused.

polyamory as a modern term is trying to differentiate from that. proponents emphasize enthusiastic consent (from all parties) and communication, and often will warn against any rigid hierarchy among the polycule members. that's an ideal, so it's certainly possible to find examples that fail, but the intent is that people are all equally respected.