When to use @⁠extend; when to use a mixin

Designer/developer/speaker Harry Roberts of CSS Wizardry isn’t generally a big extend fan:

Let me start by saying that I would generally advise never to use @extend at all. It is something of a Fool’s Gold: a feature with a lot of promise and twice as many caveats.

Compared to Harry I’m a Sass newbie, but I always found the extend call, much like nesting, mildly discomforting. This post helped me understand exactly.