My working model is thus: An adjective should be flexible in that it can describe a variety of nouns. A noun adjunct looks like an adjective but can only be connected to a limited number of nouns. So, hot oven and microwave oven show 'oven' described by an adjective and a noun adjunct, respectively. But may I apply the same method to words that appear to be adjective-like but function like a noun adjunct, and vice versa? Examples follow.
Gordian. To me, Gordian is a noun adjunct, in spite of its suffix, because it basically can only be used to describe 'knot' (see Ngram Viewer data, refresh the search to see the results, if needed)
Fermi. On the other hand, Fermi, when not used in reference to the person, can be an adjective because it can modify a variety of nouns (Ngram Viewer).
Is my reasoning sound? Do you have other criteria to distinguish these parts of speech?