When comparing Assemble vs Nanoc, the Slant community recommends Nanoc for most people. In the question“What are the best static site generators?” Nanoc is ranked 8th while Assemble is ranked 15th. The most important reason people chose Nanoc is:
Nanoc is a good choice if you need to support a site with a more complex structure than a simple blog. Nanoc is more agnostic to the types of pages you have, and allows you to do finer tuned refinements like customizing the URL structure.
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 Flexibly built
Nanoc is a good choice if you need to support a site with a more complex structure than a simple blog. Nanoc is more agnostic to the types of pages you have, and allows you to do finer tuned refinements like customizing the URL structure.
Pro Extensibile
Nanoc has a modular architecture which makes it easier to incorporate plugins and functionality from other projects as well as extend functionality.
Pro Helps you create multilingual sites
Nanoc takes multilingual sites into consideration and has features to make translations easier to implement.
Pro Unit testing integration
Nanoc has a check command to run tests against your site and make sure it meets requirements you define.
There are built in checks to validate HTML and CSS, as well as validating internal and external links.
Pro Works well with compile to languages
Nanoc is friendly with different CSS and HTML preprocessors, so you can easily use SASS, LESS, HAML, Markdown and more.
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.
