some es6 features are not understood by grunt uglify js?

You can use the harmony branch of grunt-contrib-uglify.

npm install git://github.com/gruntjs/grunt-contrib-uglify.git#harmony --save-dev

works perfectly with es6. No additional settings for grunt are required.

Officially there's no tool that minifies ES6 yet (Other tools like Google Closure compiler supports ES6 but it transpile it into "traditional js" and then it's minified).

Although not all the es6 features are supported in UglifyJs, the Harmony branch of UglifyJS2 seems to support arrow functions and other features.

You can read the discussion about it in the issue of github here. It seems there's not another option to wait (or create your own ES6 minifier).

Update

Now you have 2 options to minify ES6:

• Babili, an ES6+ aware minifier based on the Babel toolchain (beta)
• The Harmony branch of UglifyJS