From CSS Wizardry:

The problems with @extend are manifold, but to summarise:

• It’s actually worse for performance than mixins are. Gzip favours repetition, so CSS files with greater repetition (i.e. mixins) achieve a greater compression delta.
• It’s greedy. Sass’ @extend will extend every instance of a class that it finds, giving us crazy-long selector chains that look like this.
• It moves things around your codebase. Source order is vital in CSS, so moving selectors around your project should always be avoided.
• It obscures the paper-trail.@extend hides a lot of complexity in your Sass that you need to unpick more gradually, whereas the multiple class approach puts all of the information front-and-center in your markup.

See

• Mixins Better for Performance
• When to use @extend; when to use a mixin
• Extending silent classes in Sass