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.