Maybe it's too obscure, but I really like Metalsmith. Once it "clicks" it becomes ridiculously easy to make plugins and to operate, and it generally doesn't go out of date.
That said, it may require a bit of javascript knowledge to get a more customized build running. It does have YAML configuration and a set of useful plugins you can use without touching code, from what I know.
I liked Metalsmith because it was so simple that you could understand every single thing it did and why. Unfortunately, the dependency tree gets ridiculous quickly, especially since many of the plugins depend on different versions or implementations of libraries (e.g. path matching libraries). My site ended up with 200+ transitive dependencies.
That said, it may require a bit of javascript knowledge to get a more customized build running. It does have YAML configuration and a set of useful plugins you can use without touching code, from what I know.