I disagree. I find that SASS mixins provide the same reusability as classes but in a much cleaner way. True, it makes your HTML and CSS very coupled, but it does provide you with a true clean separation of content and presentation.

I find

  // style.scss
  @mixin button($color, $size) { ... }

  .download-book {
    @include button(
      $color: red,
      $size: big
    );
  }

  .send-email {
    @include button(
      $color: blue,
      $size: small
    );
  }

  // index.html
  <button class="download-book">Download book</button>
  <button class="send-email">Send email</button>

much cleaner than

  // style.scss
  .button { ... }
  .red { ... }
  .blue { ... }
  .big { ... }
  .small { ... }

  // index.html
  <button class="button big red">Download book</button>
  <button class="button small blue">Send email</button>

1. When you change the visual aspect of an element, you go to the stylesheet and don't go adding/removing classes from the content layer (HTML).

2. When using generic names like "button", "big", "small", etc... I always find myself in a naming conflict at some point. The likelihood of semantic names having conflicts is much smaller.

Of course, your CSS file becomes a lot bigger. But when enabling compression, the stylesheets using mixins turn out to be smaller (see https://tech.bellycard.com/blog/sass-mixins-vs-extends-the-d...). Also, your HTML file is likely smaller too when using semantic classes.