Nuclei runs YAML templates describing a request and the response that proves a vulnerability. Its value is the community template library and its speed: a full pass over an application takes seconds, which is what makes it viable as a CI gate rather than a one-off audit.
Its limitation is the same as its strength — it finds what a template describes. Anything framework-specific, like a Django mark_safe() on user input, needs a template written for it or a different tool entirely.