40% customization is quite high. Normally, Customation involves ABAP programming (e.g. external Interface is a typical case), Customs table (Z table, either configuration or Transaction table).
Thus, setting up of thee tables and program will take a lot of time and has to be thorough.
The more we are using Standard SAP feature, the more we will have control over the big application development. However, that doesn’t happen in reality.