Some business-scenario examples that can be achieved with SAP S/4HANA Cloud ABAP Environment, but couldn't be realized via side-by-side/in-app extensibility:
In general, everything which is tightly connected to the business process, like sales order processing, would need special coding and require more effort to do it side-by-side because you must be directly in the process. Also, in cases where a heavy data amount has to be transferred, SAP S/4HANA Cloud ABAP Environment is the perfect choice.
This table shows the tightly coupled more complex extensions for developer extensibility and loosely coupled use cases for side-by-side extensibility.
Below is a list of extensibility options.