Avoid the temptation of abstracting code into mixins. There are a few areas where mixins are perfect. The code for CSS text truncation (e.g. @mixin Truncate) or iOS style inertia scrolling panels, where there are number of pseudo selectors to get right for different browsers. Another good use case can be complex font stacks.

For example:

.med-Video {
    position: relative;
    background-color: $color-black;
    font-size: $text13;
    line-height: $text15;
    /* At medium sizes we want to bump the text up */
    @media (min-width: $M) {
        @mixin FontHeadline;
        font-size: $text15;
        line-height: $text18;
    }
}

For simpler font-stacks, a variable can handle this need easily so may be preferable. However, mixins are great for more complex font stacks, where it's preferable to have certain font stacks apply in certain situations. For example, perhaps one font is required for LoDPI, and another for HiDPI. These situations can't be dealt with by using a variable alone so a mixin is used as needed.

Ultimately, aim for ten or less mixins in a project. Any more than that and it's probable mixins are being abused to needlessly abstract code.