When comparing Assemble vs Metalsmith, the Slant community recommends Metalsmith for most people. In the question“What are the best static site generators?” Metalsmith is ranked 13th while Assemble is ranked 15th. The most important reason people chose Metalsmith is:
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.
Specs
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Pros
Pro No dependencies on Ruby, Python... just JavaScript

Pro Built on node.js

Pro Powered by a popular template engine Handlebars
Handlebars is the default template engine for Assemble, but you can add any template any you want.
Pro Highly customizable

Pro Markdown support

Pro Highly extensible
Assemble can be extended with plugins/middleware, helpers and mixins.

Pro Nested layout support
Assemble makes it easy to work with layouts. Layouts are used to "wrap" pages with common page elements, such as a header, footer etc. You can even nest layouts!
Pro Use mainstream build tools Grunt or Gulp
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.
Pro Designed around plugins
Everything in metalsmith is a plugin, and it is designed to make it easy to write new ones.
Pro Chaining API
Metalsmith uses a chaining API that's consistent and simple to use:
Metalsmith(__dirname)
.use(markdown)
.use(templates('handlebars'))
.build();
Cons
Con Documentation can be hard to navigate
Especially for someone new to Assemble, it can be difficult to find what you're looking for in the documentation.
Con Small community
The Metalsmith community is still fairly small compared to the more popular options. This results in a lack of learning resources and difficulty finding support from experienced users. However, a slack group has recently been started.
Con Not client enabled API chaining
According to the spec on API chaining, the API request/response need to associate an API object to an corresponding controller/action/uri to be able to validate and handle the request/response and the datasets.
Without that, the consuming client service cannot properly validate or relate the datasets from one to the other. Links have no relational value and the api object creates the relationship between the datasets while maintaining an api/dataset relationship with the corresponding controller/action/uri
As such, the current methodology supported would not be able to be called by the client not supported by the client in a RESTFUL manner
