It was spooky and magical because it worked so well and it was based on symbolic transformations -- not mathematical ones. It'd be liking having calculus' basic

It was spooky and magical because it worked so well and it was based on symbolic transformations -- not mathematical ones. It'd be liking having calculus' basic derivative rules without their proofs. Without the rigorous backing, Umbral calculus was used for research and exploration but then additionally work had to be done to prove the results. Now, with the rigorous backing, the results stand alone - as the umbral transformations have been proven to be rigorously valid. I am not too knowledgeable on the proofs of validity of Umbral calculus but my guess would be that only part of the vast library of umbral tricks, hacks, and symbolic bashing have been formalized - so the rigorous subset of provably correct transformations is likely more limited and thus cumbersome.