Add comment with Inline Entity Form while adding content
Inline Entity Form doesn't support Comment forms out of the box. 

Part of the process of building the IEF widget is a call to inline_entity_form_get_controller() which will retrieve a controller class from the entity metadata. As you can see in IEF's hook_entity_info_alter() invocation, it only adds this controller metadata for node, taxonomy, commerce product, &amp; commerce line item entities. 

Theoretically, you could build your own custom module that could:


Invoke hook_entity_info_alter() to add support for comment entities.
Build your own custom class that extends EntityInlineEntityFormController(e.g. CommentInlineEntityFormController) to be evoked for form processing.


Keep in mind this is an unusual use case, a comment entity is predicated on a its parent entity pre-existing before it's created; you're likely to run against logic that might prevent this (e.g. database integrity checks) . 

It might be easier just to create your own custom node type entity that's configured just to appear like comments on your nodes.