Converting Ember addon to ember engine

When converting any Ember addon (say my-custom-app) to an Ember engine, what all things does it loose like ? Meaning since engines are more isolated, does it loose ability to be easily shared e.g. in case of addon, app directory files just get merged. Does the same happen with engines ? Also what about things in addon ?

In short, just wanted to understand what happens in an addon environment V/s an engine environment ?

PS: I am not referring to an in-repo addon here