@miroslav-lee
As already mentioned in the module's description-

MMF is not ...

@miroslav-lee

As already mentioned in the module's description-

MMF is not implemented for below fields as they have different meaning to 'Allowed number of values: UNLIMITED'.
In other words, widgets to these fields do not have ADD MORE BUTTON.
* Select List
* Check boxes/radio buttons
* Autocomplete
* Autocomplete (Tags style)
* Single on/off checkbox

Entity reference field widget indirectly belongs to the above group of fields, hence did not implemented.