Finish is, in many respects, a necessary evil. You have to do something to protect the wood from dirt, grease, and wear, but any finish adds mass, which we try to avoid. Finishes that contain oil also add damping, which dissipates sound more quickly, especially in the higher frequency range. Since the problem with Classical guitars is the intrinsic lack of highs in the strings due the the damping of nylon or gut, it makes sense to avoid adding any more than you need to with the finish.
Shellac and lacquer have low damping. Of the two, shellac is more stable over the long run: nitro breaks down, but shellac cross links and gets stronger and more chemical resistant. Sadly, it takes a long time for shellac to really become impervious 75 years or so. Shellac when it's new is soluble in alcohol and in alkaline water solutions, which seems to include sweat for many people.
Traditional oil-resin varnishes do have some oil in them, and thus have higher damping that shellac. 'Spar' or 'long oil' varnishes have more oil than 'short oil' or 'rubbing' varnishes. Many of the latter don't add much damping at all, so far as I've been able to find out. The oil content does contribute a lot to solvent resistance, and many of the better varnishes are quite stable over time, and remain flexible and tough.
The main thing to pay attention to is how much finish you end up with on the surface. A good French polish job will be .002" thick (.05 mm) or less. It's difficult to get a good coating of anything else at that thickness, but some oil-resin varnishes can end up at .0025"-.003". It's hard to get a spray finish such as lacquer thinner than .005", and many are much thicker than that. I've seen water based finishes that build to .008" or more. The 'prize' there is the finish I measured when I replaced the top on an Ovation guitar some years back: it was epoxy, .04" thick, a full millimeter, and hard as a rock.
The bottom line IMO is that 'less is more', so long as it provides the protection it's supposed to. In high wear spots FP makes little sense, especially in places like the neck that have very little bearing on the sound, if any. FP on the top provides the protective coating you need, and can be durable enough if you're careful. Even the slightest touch can mark it on a cedar top, though; spruce is a bit better. A slightly thicker coat of a good varnish might just be enough more durable to be a good trade-off with minimal impact on the sound.