What do you mean by "poorly documented"? I have been using it for 20 years, I have yet to find something that is not documented.

As for convoluted, I don't find it harder than the other build systems I use.

Really the problem I have with CMake is the amount of terribly-written CMakeLists. The norm seems to be to not know the basics of CMake but to still write a mess and then complain about CMake. If people wrote C the way they write CMake, we wouldn't blame the language.