The problems GNU Emacs does not solve for a CL programmer is its generally clunky user interface, lack of GUI features, its non-reusability in a Common Lisp program and its non-source-compatibility with Common Lisp. GNU Emacs will always be an extra component to a CL program and rarely will be the right user interface for a CL program - even though the program might need editor functionality.

One can get very far with it, but for some the combination of above is enough reason to look for alternatives.