having one relationship working is already pretty complex
Fully agree.
Today's example, I don't know, but the guy I talked to last time didn't seem to be that happy about the polycule. It seemed that he kinda accepted it as it was his girlfriend's desire, but he'd rather have her be exclusive. So, by definition, it's already complex, and to have a balanced one where everyone is on equal footing is likely to be even harder.