Creating a custom popup field that derives from BasePopupField or PopupField is limited because the 2 functions GetValueToDisplay and AddMenuItems are marked as internal and so cannot be overridden outside of internal assemblies. 

If a user wants to customize the displayed values they have to override ToString which is not always possible or simple. 

For example:

I want a popup list that displays a selection of types that inherit from a base type. I wanted to remove the namespace from the labels and just show the class names. I was able to do this for the selected type by overriding the value property, however, there is no way to get to the menu items and so my popup shows the full namespace for the items but the cleaner value when selected.