While the usage is established, I would not call them adverbial or adjectival clauses, in that they function differently from adverbs and adjectives in general. They tend to come after the noun they qualify. They also do not act like adverbs and adjectives. (No comparative or superlative forms. etc.) I would go with Huddleston & Pullum's (Cambridge Grammar of the English Language) terminology and just call them complements and adjuncts,

Ceci n'est pas un seing.