All of the "classical sins" could be described as are human nature regardless of relationship type. Note that as an alternative I don't just think of flat hierarchy polyamory here, any non-heteronormative relationship or constellations that revolve around a "main relationship".
In a world of industry standards, automatization, light and flexible work hours, easy communication and high mobility I think we need to be creative again. Especially how this above evolution seems to reach its limits, and "scale to the moon" does not seem to work (dead internet) and we're in dire need of small, informal communities again. I see much more openness to this from the younger western generation.
For everybody else, there is the normal and perfectly human feelings of jealousy, attachment, fear or loss, and feeling associated with self-confidence.