Figured bass represents upper voicings against a bass note, not against a root note or a key note. In this sense, it is not harmonic as we know the term.
Figured bass classifies voicings, not chords.
Figured bass classifies chord voicings by groups: all seventh chord voicings, all 6/4 voicings, etc.
It does not distinguish or group "all C chords" because there is no reference to a root or the key. Roman numerals were used only later in figured bass.
Bach's system did not recognize chord inversions as being the same chord. It did not recognize chord function, or use chord progressions.
So "where's the beef?"