The Pit Vipers

With this argument, you also strongly advise against buying speaker kits.
Not necessarily, although some driver to driver variation is to be expected. Ideally you'd measure the results and if necessary tweak the crossover.
According to this, even ready-made loudspeakers would have to exhibit extreme fluctuations in measured values if they were not developed individually, specimen by specimen.
I'd expect commercial manufacturers to do quality control, and where necessary either adjust values or reject out-of-spec examples.
these extreme fluctuations in measured values do not occur
I never said the variations were extreme.