Currently, the configure board side bar lists all the units of all the card types that are selected as cards to be shown in the board, but doesn’t allow selecting those units which are not common across all the cards.
A more flexible approach will be to allow showing any card unit whether it is common across all cards or not. In fact, it is quite essential to do so for the very purpose of creating multi-type boards.
For example, this will help a lot in the Activity Streams view I created. Like currently, I can’t show assignee avatars because not all the types have assignees.