I agree with everything you said about other aspects of Emacs.

You are right that supports for C++ is not as rich, since it is such a complicated language. But for small C++ projects and large C projects, it is pretty much usable, especially just for code navigation. At least it's better than using Grep and better than nothing at all. The currently available tools are perfectly usable for those who only write C.

For code completion, there's no pretty solution, but at least simple solution like company-clang which uses clang to get completion candidates from system include paths is pretty usable.