One problem is, for a circuit of some complexity, the
parasitics are distributed and can combine in many
ways (in the same circuit). You can have shunt and
feedforward, in various places and proportions. A
single-pole or few-pole, or few-poles-plus-zeroes,
might be discernible in the details from a gain/phase
vs frequency plot (if you have the schematic in hand)
but how many can sit on top of each other and still
let you pick them out of the tangle, quantitatively?