The main reason for migration was that I wanted to write my posts in a lightweight markup language such as Markdown or reStructured Text. In Blogger I had to either use a WYSIWYG editor which is OK for simple posts but not for advanced stuff such as code snippets or formulas, or an HTML editor.
GitHub Pages have additional advantages:
All the posts and the change history are stored in a Git repository which usually exists in at least two places, on a GitHub server and locally. Each can act as a full backup.
There is a clear mapping between the content of the repository and the site. Even though I am using it for just over a day I can open any of my posts in a browser, click View Source (HTML) and say where every last piece of HTML has come from. This makes configuration simple, if I need to change anything I immediately know where to go.
Now I can use the same tools for writing my posts that I’ve been using for development.
There are some disadvantages as well:
You have to add yourself many of the things that are readily available on Blogger: comments, analytics, tags. Fortunately it is not that difficult.
Error messages could be more informative:
The page build failed with the following error: page build failed