The problem with Gulp is just that it just adds too much extra cruft into the mix that isn't really necessary, mostly around running tasks from the CLI. Our general thought for build tasks is that all of that should be in a Makefile which will nicely handle mtime checks and everything for you, and is available on pretty much every setup out there.
The simplicity is nice because you can read through Metalsmith's source and really understand everything that it's doing very quickly: https://github.com/segmentio/metalsmith/blob/master/lib/inde...