You definitely need a comparison with webpack. As far as I can tell from a quick scan, webpack has all of the capabilities that brunch does. More importantly, it's much more popular, so the plugins and help you need are likely to be available.
[1] https://github.com/brunch/brunch-guide/search?utf8=%E2%9C%93...
Instead of telling why it is better than Grunt or Gulp, I'd like to know how it compares to webpack (or browserify, which seems to be quite close to webpack in functionality nowadays, http://reactpodcast.com/2015/06/webpack-vs-browserify/)