As the others have mentioned, oil-based varnishes need some mechanical help to bond to shellac, or even to themselves. Thus the caution about dewaxed - not to the point of foolish - shellac and a solvent wipe after "toothing" the shellac with sandpaper as preparation.
Long-oil varnishes, and they can be made with any of the curing oils, are used outdoors because of their flexibility in response to heat/cooling. Traditional oil used is linseed, tung being a relative late-comer to the western world, whereas flax (linen) was grown all over. Virtue of phenolic resin versus urethane with UV inhibiters is a matter of opinion. The modern stuff is probably good enough, maybe better than its predecessor, the first synthetic resin. Key is that the wood will continue to move with water uptake and loss, film will continue to move with temperature variation in opposition, and the finish will probably peel before it yellows and clouds unless it's living in full sun, something I avoid on my patio. That's the reason why you see a lot of plain oil renewables as "finish" for outdoor wood. That'd be my choice.
Shellac, since it does not share a common solvent with resin, is a great choice for trying to seal knots in softwood. Since we turners tend not to use kiln-dried (and resin-set) wood, it's the only countermeasure we have. You can bake your turnings for resin setting, I suppose, but that brings another problem set along with. Just as outdoor use does.