Working with this project, which extensively uses @at-root, I found some differences between the result of scssphp and SCSS compiler. This is one use-case:

.container {
  @at-root body#{&} {
     #logo { 
      border: 1px solid red;
    }
  }
}

The expected result, as shown here

body.container #logo {
    border: 1px solid red;
}

The scssphp result:

bodybody.container #logo {
  border: 1px solid red;
}