The makers of the guitars I play have their own special formula regarding varnish. I have played many old guitars which have French polish end, and although this kind of finish is regarded generally as a high quality varnish, it does not really protect the guitar either for hot or cold climates. It was used for many reasons but the main one was because many years ago there was no other better way to protect the wood of the instrument.
French polish, although it looks “beautiful”, is not very durable or practical for players who often tour, play and travel to different climates, exposing their instruments to airplanes, air conditioned in recording studios etc. French polish can be worn out even by sweat!
The varnish of the guitars I play is a very good one because it allows the sound to flow, hence it does not cover up excessively the inner texture of the wood and protects the guitar as well. It’s similar to the lacquer one but much finer, a superior quality. On the other hand, as far as the mixture of natural /chemical elements is concerned that kind of varnish is something in between the French polish and regular lacquer.
For a luthier every single aspect should to give predominance and paramount importance to the sound and durability of the instrument. Regarding varnishes there are hundreds of varieties out there and therefore functionality and sound are first in my opinion.
In addition, this video illustrates about varnish materials for guitars: