What are the pros and cons of using an oil medium or liquin instead of
a traditional varnish on a finished oil painting?

With regards to using Liquin, the downside is that it cannot be
removed with current known materials once dry. Varnish can be removed
with mineral spirits and other oil solvents. Liquin dries slower than
varnish resulting in the need to keep the painting flat for a few
hours. This extended horizontal time may result in airborne particles
settling onto the wet surface thereby trapping them in the Liquin.
Varnish and Liquin both have this feature but varnish dries more
rapidly allowing one to position the painting vertically quicker or
surface the painting in a vertical position. The vertical position is
problematic only to the extent that sags on the surface are possible
when the varnish or Liquin is wet. If varnish is used it is considered
good practice to apply it continously to the surface until it starts
to become, "tacky" and thus will not run or sag. The good stuff is
that Liquin is far easier to work with than varnish and produces less
vapors. It also presents a dried surface that allows one to paint
directly on top with no reactivation of the medium once it is dry.
Liquin is an alkyd and tends to resist yellowing to a greater extent
than varnish so say the makers. Liquin may also be thinned so as to
create a reduced level of gloss. A mix of 50% mineral spirits with 50%
Liquin produces a slightly semi gloss surface. Additional coats may be
added once dry. This adds more gloss. The cardinal rule of varnish is
that it be applied thin. Liquin also has a higher degree of resistence
to water droplets should they get onto the surface once the Liquin has
dried. Varnish easily spots from water creating milky areas that are
difficult to remove. Liquin provides a very strong bond to oil paint
and many use it during the painting process itself. Perhaps this is
the major reason for its invention. It is suggested to be a very good
medium for glazing and detail as it increases the flow properties of
the oil paint.