I think that its more difficult for a stable, persistent, nonmonogamous, romantic/sexual situation to persist mainly because there are more people involved.

Everything that would be a one to one discussion, is now A to B and A to C and B to C, and potentially A to BC and AB to C and AC to B... this gets more complex, geometrically, with more members.

With more people and no mandatory/imposed hierarchy, It complexifies, with more chances for miscommunication, with all the intensity of emotions that comes along with a serious relationship... which can often lead to drama.

I don't think that this is conceptually difficult to do ethically, if everyone involved communicates very well.

But that almost never occurs in practice, in mono or nonmono setups.

I think it is difficult to do ethically in practice, moreso when there are more members, because people have emotions that cause them to do irrational things, they have limited amounts of time and energy, imperfect information, because people can change their minds about things, because sometimes people don't really know why they do some things.

The more people you have in a persistent arrangement like this, the more complex and thus unstable the entire situation is.

Granted, that reasoning only applies to certain kinds of non monogamy, others are or can be less complex...

But basically my whole thrust here is that more people = more complicated = more chances for drama / intentionallly or unintentionally hurting other people.

There are just more potentially shifting sets of boundaries and rules, that may or may not apply equally to all others in the group, and those boundaries themselves may or may not be problems for other members of the group.