I've been told that, before KDE switched to CMake (becoming the first big open-source project to do so), there were only two or three people (out of hundreds of developers) who dared to touch the autotools stuff, except for peephole edits like adding a new source file to a list. For everything else, one of the experts needed to be brought in.
I only came into KDE when the switch to CMake had already happened, and remember it as reasonably approachable (even if quirky). Most developers were familiar with it and actively authoring the CMakeLists.txt files for their own projects. (That doesn't mean that there weren't some experts again, but they focused on implementing reusable modules that the others could easily integrate into their own build system.)
I only came into KDE when the switch to CMake had already happened, and remember it as reasonably approachable (even if quirky). Most developers were familiar with it and actively authoring the CMakeLists.txt files for their own projects. (That doesn't mean that there weren't some experts again, but they focused on implementing reusable modules that the others could easily integrate into their own build system.)