Drupal 8 Custom Entities from a UI?
Each entity needs a lot of classes for access, forms, storage, listing, viewing,  etc... so you would have to create a module with default classes that would be somehow generic and you would have to handle the discovery of these new entity types since D8 uses annotations. Overall a quite a lot of work.

The same goes for config entities but that would be easier since they do not need that much handling compared to content entities.