To clarify a point, shellac was heavily marketed as a sealcoat. It's not really a "hoax". Back when home woodworking became popular, shellac was pretty much all you had to work with as a hard, durable finish coat. Then came along a whole host wiping varnishes like Formby's tung oil finish (contains absolutely no tung oil), Waterlox, etc. which were much easier to work with. To stay relevant in the market, Zinnser pushed the seal coat properties and aspects of shellac. Rather than try to compete with the huge storm of new products people were snatching up, they pushed their product as complementary to varnishes; not an alternative or replacement of - which shellac essentially is.
I was always taught (and just read a book that reinforced this) that the shellac seal coat is not as necessary as people make it out to be. There are a few uses where it excels (I am talking about shellac as a seal coat, not a finish coat):
1 - before waterborne finishes (as mentioned)
2 - To seal off overly oily woods or sappy knots (like in pine)
3 - Odor barrier used when refinishing (pet stains, smoke, etc)
Other than that, your first coat of finish is your seal coat. Shellac is just another finish. It works well as a barrier for the list above, but any other finish will do just as good of a job sealing the wood. I've never seen this happen personally ,only in pictures, but because shellac dries SO HARD it is possible to crack it under your actual finish with some concentrated force.
In addition, dewaxed shellac offers much better moisture resistance than the waxed variety. Since shellac has a short self life, if you use it a lot it is cheaper to cut your own.