Ross' musical talents are not as amazing as he and Sam think.
In the original sentence (above) you have a fairly complex adjective phrase which contains a comparative clause. You have here a comparison of degree.
Ross' musical talents - subject
are - linking verb
not - adverb
as amazing as he and Sam think - subject complement - adjective phrase in the form of a comparative structure
as amazing as he and Sam think :
as ... as - the framework of the comparative structure (adverbs of degree)
amazing - the adjective highlighted in the comparison - head of the adjective phrase
he and Sam think - the comparative clause
he, Sam - subjects
and - the coordinating conjunction to join the subjects
think - the verb of the comparative clause
The underlying statement is complex:
as amazing = equally amazing = amazing to a degree equal
as = to the degree that
Ross's musical talents are not [amazing to a degree equal] [to the degree that] he and Sam think (that Ross's musical talents are amazing.)
CJ