I have never bought a mold (from Etsy or elsewhere) however I do make my own, and generally stuff comes out looking how you'd expect. If it's a particularly large piece, there's a good chance it might warp as you try to remove it uncured, just because it's not yet solid. Therefore I make all my molds so they can survive in the oven, and remove the contents once it's cured if necessary.
The only problem I find is eventually the molds crack with age, especially if you've been baking them,