Единственная запара возникла при подключении иконок к рельс6, решается так (app/javascript/packs/fa.scss)
@import "bulma/bulma";
$fa-font-path: '~@fortawesome/fontawesome-free/webfonts';
@import '~@fortawesome/fontawesome-free/scss/fontawesome';
@import '~@fortawesome/fontawesome-free/scss/solid'; // fas prefix
@import '~@fortawesome/fontawesome-free/scss/brands'; // fab prefix
// see https://fontawesome.com/how-to-use/with-the-api/setup/importing-icons for more prefixes