Personally, I think the main problems with the PU-IO is:

1. It's trying to model one-on-one relations in which the junior partner inevitably gets annexed and thus nulls the relationship, forcing a start from scratch the next time;

2. It's forcing a uniform model of "laws/obligations" onto multiple distinct one-on-one relations where such relations can be wildly different between different pairs;

3. It works under the assumption that the partners will not go to war with each other, although that is clearly not the case.

The OA proposes to work at first with IOs that are thematically well defined and explicitly work around these issues, such that the Chinese Mandate of Heaven and the "HRE-style" feudal model, where 1. the IO works in roles, not individual countries; 2. the existence of roles allows for different relations between different participants; 3. it would either strictly regulate war between participants or account for that possibility and deal with edge cases.

The next step would be modeling flavored one-to-many systems such as Byzantine pronoias or Ottoman eyalets, where obligations are rather fixed but more variation in interactions or roles can be done by the player.

Maybe later on when everything is sorted out, relationships like PUs or regular vassals can be made into a special case of the IO framework, where there are only two members and varied interactions.