Its source code is a maze of twisty little passages, all alike. Its build system is an abomination from the deepest circles of Hell.
Also, pure C for compilers is just a dumb idea. They even had to write a garbage collector because it's impossible to write anything complex in C reliably.
In comparison, LLVM development is just a breeze. First, I can use CMake to generate a VisualStudio project (I still haven't figured out how to build GCC using MSVC). Then I can easily write front-ends with LLVM. A simple non-trivial codegenerator can be written in about one day.