Does anyone know away to create custom entities from Drupal 8 UI that is not ECK? I try it out using ECK and still a little bit buggy.
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.
I don't think it's possible right now and the way to go is wait for ECK port to be more stable.
It's a lot of work to do and ECK maintainers probably are the ones closer to do this.
The other approach is to use Drupal Console, that is really easy to use.
Drupal 8 is not yet too friendly for site builders, in my opinion, as things are not stable. However, for developers, it's already awesome.