This webpage outlines the "four grammatical forms that can function as the adjunct adverbial". In the tree underneath, I agree that "aside and "ago" are postpositions, but please see the question in the title.
Why isn't the red node (not the one underneath) a Verb Phrase?
Why isn't this orange node a Noun Phrase or Determiner Phrase?
In Adverb Clause as Adjunct Adverbial, isn't the Clause categorized as Adverb as "unless" is the first word to appear in that Clause?