When everyone is too invested in everyone to the point where it's clear they don't have much life outside the polycule. When one member of the polycule is clearly in charge of it or is taking a significantly disproportionate amount of the time/energy/effort. When it's clear that individual relationships within the polycule are expected to take a backseat to the group relationship. When there's backstabbing, gossiping, or anything similar within the polycule. When metamours intrude on conflict between partners. I'm sure there's more.

These types of polycule are unfortunately not super uncommon. Internet rationalists imo are the worst for it, but they're by no means the only ones doing it. In general just watch out for relationships that look/feel more like a commune/cult than they're upfront about being and make sure that if you do enter a more communal relationship dynamic that they're keeping safeguards to make leaving accessible for anyone who wants to.