It has worse code completion and refactoring support for C and C++ than it should have. Emacs users are forced to switch to other tools (like proprietary IDEs or LLVM based tools) when they need to do complex refactoring to c++ codebases. It could be improved (should have been years ago) but RMS has stated that he would block the inclusion of code to do those things if they use a GCC plugin that exports the full AST.

This is all discussed in the original mailing list thread and it's why one of the core emacs devs suggested forking.

See the thread that prompted this: https://lists.gnu.org/archive/html/emacs-devel/2015-01/msg00...

Basically, an Emacs maintainer thinks that this gcc restriction is so harmful to important emacs features that he would be willing to "consider a fork"