The testing I've seen shows nitro as a harder finish than shellac, but also more prone to chipping and scratching. Shellac is tougher because of the way it is cross linked; it's more likely to dent than scratch.
Many years ago I did a science project on plastic, and one of the ones I made was nitrocellulose (imagine!). You can nitrate cotton to various levels; at about 18% nitration it becomes soluble in a range of very nasty things like MEK. In use they limit the amount of these by cutting lacquer thinner with somewhat less toxic things. The least toxic ingredient in most lacquer thinners is toluene, and that's toxic at a concentration of 1/10th the level you can smell.
Highly nitrated cellulose is a high explosive: each molecule contains everything it needs to 'burn', and it only requires a bit of a kick to start it off. UV light can supply that. Even the low level nitrated stuff we use as a finish is 1/6 high explosive; not enough to actually blow up, but it does burn enthusiastically, and suffers from the same lack of stability in the long term. A student of mine who worked with museum conservators said that they rate a nitro finish as having a useful life of about 75 years. Given that a well made guitar can last for well over 100 years, why use a finish that will be flaking off long before that?
Several years ago the Boston Museum of Fine Arts assembled an exhibition of guitars as art objects. It was easy to see in that show when the Martin company switched over to nitro; the earlier guitars that had been finished with shellac and varnish looked much better. There was a Stronberg arch top in the show that had a tick nitro finish, along with celluloid (the same material) for trim. The bindings were cracked every inch or so, the headstock veneer looked as though it had been burned with a torch, and chicks in the thick finish were continuing into wood. It's no wonder they left that out of the catalog: there was no way to get an attractive photo of what was otherwise a fine looking instrument.
Varnish is not a panacea, of course (no finish is). 'Run' copal varnish turns black overt time, and rosin varnish shrinks and cracks badly.
If it were not for the initial issue of wear shellac would be the best of them all. Sadly, it's soluble in alkaline water solutions, which seems to include some folk's sweat. The solubility decreases over time, so that after about 75 years it's impervious to almost everything. It's hard to get the customers to wait that long.