All the Fine-N-Scale models are proof that the process works when you start with a great master.
Absolutely. The resin casting process is so faithful that it will even reproduce fingerprints left on the master. It must be as clean and precise a build as possible; casting cannot possibly improve a bad master or hide any flaws. A secondary aspect, but one that's still important, is the mold: a lot of times people will make a mold that's too small and flimsy, which allows it to flex; this is an especially important factor for long objects with thin parts, like coaches and loco shells. A big beefy mold will yield better castings.