I strongly suggest to give admin users the ability to see and do everything with every board (except private boards of course).
At this moment, an administrator cannot do his work, since he is not aware of the existence of shareable boards, unless someone subscribed him to them.
Once subscribed, he may be an admin, but still cannot change board subscribers and change everything he deems necessary, unless he is assigned ‘board owner’ by the registered board owners (who may be users that have left).
With the current permissions, he needs to be subscribed to EVERY board to be able to administer it, meaning he receives tons of updates and notifications about things he’s absolutely not interested in. Updates and notifications where he is interested in, easily get lost in the bulk of ‘trash’.
And all of that, only because an admin doesn’t have true admin rights at the moment