Assume you have a .kateconfig file optimized for C++ code that replaces tabs on save. Now you open a Makefile that contains tabs (due to its strict syntax). If you save the Makefile in kate, the tabs then are replaced which results in a corrupted Makefile.
In other words: The .kateconfig file applies to every file. It lacks mimetype/extension support and thus can lead to unwanted behaviours.
So if you use a .kateconfig file, keep that in mind :) Maybe we should simply use modelines in the h/cpp files for now.