It's awful. It's not even a build system; it's a framework to make your own custom special snowflake build system that is unique to your project and which contains 1% of the functionality you get for free with cmake.
Take a trivial problem: enabling threading in a cross-platform manner. CMake: "find_package(Threads)" Autotools: "ACX_PTHREAD" SCons: Write it yourself.
Now multiply this by every single other detail of your project. It's wheel reinvention writ large, and no one else will have a clue how to build or work on your project.