I just found out there is one more behaviour to be aware of in the M2P. I have 3 levels of membership. The main one, a paid membership (1), two more that are manually assigned in addition to (1) for board members (2) and administration (3). ALL have a paid membership (1), a few have additional memberships.
You would think that assigning content to a membership that all members belong to, i.e. membership (1), would display even when members have additional memberships, (2) and/or (3) AS WELL. This works for pages, custom views, but not for Categories.
Categories which are protected by membership (1) show for all members who only have access to membership (1). But if a member has access to membership (1) AND (2) categories don't display. The only way to make this work is to assign protection rules to Categories that include all memberships.
This is a comment not a support request, but you should aware of this.