- Extensibility. (definer doesn't support extensions.)
- Affordable dependency. (cl-def depends on some libraries, and those libraries depend on some other libraries and so on... You're required to install a dozen of libraries to benefit from a single definer utility. Furthermore, a major portion of related libraries are not available via ASDF-INSTALL.)
To summarize, demacs is implemented with KISS principle in mind. And as a bonus, demacs has no dependencies.
For details (syntax, extensibility support, editor indetation stuff, etc.) README.