Poly is the most commonly used clear "varnish" used these days. Most so called "lacquer" finishes are actually polyurethane. True nitrocellulose lacquer involves fairly nasty solvents and requires special permits to use. You can't do it at all in the SF Bay Area.
Polyurethane takes an extended time to completely cure. 6-12 months. The apparent softness of partially cured poly will have a deadening effect. Thick coats of poly look great but have developed a really bad reputation in the musical instrument world because it initially deadens (very slightly) the instrument compared to real nitrocellulose lacquer.
Hard finishes on the inside of a shell are a mixed bag. You get more apparent volume and improved projection. But you can also get that weird sewer pipe effect that is hard to tune out. Some things like the the Zolotone in old Rodgers are a mix of sealing the inside of the shell and making it harder, but not making a smooth reflective finish.