The bar is low for such tools regardless of programming language. In a language as dynamic as Ruby it's several miles into the Earth's crust. The tool won't be able to tell you much of anything you shouldn't already know. "Potentially high-impact web vulns" is a next to useless metric when provided by such a tool. The rate of false positives is high. A distraction such as this when your application surely has more serious vulnerabilities is not helpful.
Railroader and Brakeman compensate for this by not being generic analysis tools for Ruby, but instead focusing specifically only on Ruby on Rails. Because Ruby on Rails has a lot of additional conventions, it's much easier to build a specialized tool to look for violations of those conventions.
