sonar-scala is developed in Scala. It uses the scalariform library to parse the source code and integrates with Scoverage (code coverage), Scalastyle and Scapegoat (static code analysis). It also provides pull request decoration functionality, which can review pull requests on Github and make comments on new issues directly in the pull request instead of reporting them to SonarQube.
The major advantage of sonar-scala over the official SonarScala plugin is that it's built around tools such as Scalastyle and Scapegoat at it's core. It provides built-in rules and quality profiles for both of those tools, which allows you to easily explore those rules and get an aggregated view of existing issues across all of your projects. Scalastyle is built right into the plugin and doesn't require any additional set-up on your end. Additionally, sonar-scala supports pull request decoration on GitHub, for which SonarScala requires either a paid Developer Edition of SonarQube or SonarCloud subscription. sonar-scala is free and will always remain free!