my experience some years ago, wen i was studing the guitar making, i have finnished one of my guitars with nitro, it was my first portuguese guitar.
unlucky wen i send the guitar to my client, i was in Portugal and he in Luxembourg , the lovely logistics enterprize has damage the guitar, i believe the instrument was in a very high temperatures, then the finish was brake in some parts and one of the sides as well.
my cliente kept that guitar for some 2 or 3 weeks and wen he have the chance he decided to visit me and repair, at that time we decided to strip all guitar and apply shellack
the diference of sound was really amazing the guitar sounds really better then.
no in this days i can tell that this fact decided my future works, i now work with shellack and oil as well, i love the oil finish, but to use on soundboard you must be very careful and must build very slowlly.
Like lutes, in some tradicional instruments in Portugal we use only bee wax, is a orange wax you just spread a very tiny in top the thing is, we must do this time to time, for a guy that plays every day maibe 1 tipe year. This kind of finish the musitian must have some attention with the instrument, but i believe with all finishes must have carefully and atention.
i believe that for back and sides we can use other varnishes but will depend what we wish, the poly is very hard and the finish looks like plastic, but if you like thats ok, affects the sound? i believe that is does, thinks this way, in the smallman guitars the objective is block back and sides, then in this guitars is totally understandable use polyurethane... and now existes the water version for the guys that like me wishes to protect the enviroment that is a good thing
I saw that lots of builders at this time are using spray varnishes for back and sides and shellck on top... that can be a good choice as well..
P.S. in my opinion the oil finish for the neck is a very good choice, the touch is smooth and does not make noises wen you move your hand like a high gloss finish...