Why so much limitations on customisations?

This is my opinion. SAP had a large market share in the traditional ERP space. Part of this was because once it was implemented it work very well and customizations were bolt-on functionality. With Cloud Apps/Fusion, Oracle wants to make sure that the base code stays at a level that they have complete control over. This is to ensure peak performance and consistency of the applications between environments. This also appears to be a similar approach to that of SAP. They are still all for the ease of creating customizations on top of their base code to enhance and meet their customer specific requirements. They have just become more protective of their core functionalities and private packages/tables/workflows. Larry Ellison said during on of his keynote's at OOW 2018 (paraphrased), "Customizations are good and we still want you to create them and use them as needed, we just do not want you changing our core code and functionality. But please, build on top of that code." So, the limitations are there in their efforts to standardize and streamline their base code in order to allow for additional enhancements and functionality like their autonomous functions in the future.