What are some alternatives?
When comparing Assemble and Nikola, you can also consider the following products
GatsbyJS - Blazing-fast static site generator for React
Wintersmith - Flexible, minimalistic, multi-platform static site generator built on top of node.js
Hexo - A fast, simple & powerful blog framework, powered by Node.js
Metalsmith - An extremely simple, pluggable static site generator.
Nanoc - A static-site generator written in Ruby
Jekyll - Jekyll is a simple, blog aware, static site generator.