Currently when someone uses an Entity Reference field to reference nodes of more than one content type, there is no good way to distinguish one type of node from another. For example there may be "Season" nodes and "Person" nodes referenced by a single ER field, yet there could be more than one node titled "Autumn" or "Summer" (both of which can be both a person's name and a season).
Since entity reference field already has a "Sort by" field for the Simple entity selection, why not allow rendering that sort criteria as a Group heading to distinguish one group from another. A simple checkbox "Render Group Headings" or something similar could act as an on-off switch.
Here's how ER field currently renders:
Here's how I'm proposing it should look: