Static code analyzers don't seem to find too much interesting in my experience. They could basically only find string interpol in queries and uses of constantize. Ruby is just too dynamic to find any really interesting issues.
Really? I find that brakeman is a pretty amazing tool which finds a number of surprising issues. Of course, these days the vast majority of Rails apps already have brakeman set up, so it's used more as part of the commit process and less of a "wow, here's a few dozen potentially high-impact web vulns". I wouldn't hesitate to say that it's the most high-signal SCA tool I've used across any language/framework.
(source: a few years of webapp pentesting and Rails app dev)
I ran brakeman on our app and it found a bunch of things that were almost vulnerabilities because it was unable to work out the source of some data going in to a potentially unsafe function but after I inspected all of them, none of them were actual vulnerabilities. Meanwhile a bunch of real issues go undetected that could have been spotted in a language like rust.
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.
