Actually neither is perfect but both are good enough at harvest as far
as I am concerned. Both calibrations rely on pure sucrose; niether
scale nor device has a fudge factor added to compensate for the other
things found in must. That said, it's not important to compensate for
them. The values considered optimal at harvest were arrived at
empirically a long time ago and the most important (to taste) other
solid involved is the acids. Ther are good ways to determine the acid
balance at maturity too.

Measurement technique affects both devices; I would not say one has an
advantage over the other. Both have well known limitations that should
be considered. I don't use a refractometer personally but would if I
grew the grapes myself. I would still use the hydrometer too, but
that's my way.

Joe