Pro Clean, easy to understand architecture
The entire Metalsmith core is currently ~350 lines of code in total, and the system is built around the simple idea of manipulating a dictionary-like set of file objects. This architecture makes writing plugins and understanding the Metalsmith code base super-easy.
Pro Flexible beyond a static site generator
Because metalsmith is at its core focused on transforming directories of files, it can be used for more than just static site generation, and could be used as a build tool, a documentation generator, or any use that requires file transformations.