Honestly, I think those would get easier rather than harder. If you no longer have a set limit on ‘roles’ in the marriage, you don’t have cultural intuitions about correct outcomes, and designing a system that is simple and easy to execute gets easier.
On the contrary, I think divorce and child custody would be the thorniest issue (as it is in binary marriage divorce).
Honestly, I think those would get easier rather than harder. If you no longer have a set limit on ‘roles’ in the marriage, you don’t have cultural intuitions about correct outcomes, and designing a system that is simple and easy to execute gets easier.