I've been using Rails for over a decade and worked on lots of different Rails code bases. Contrary to what you're saying, my experience is that the philosophy of "convention over configuration" usually makes it very easy to delve into a Rails project you don't know, as long as the developers didn't decide to go off the rails with experimentation.

Then of course you can find badly written code in any language, and if a quirky would-be rockstar creates a project thinking about art instead of readability and maintainability, they can definitely create an unmaintainable mess with Ruby on Rails - there is nothing preventing you from doing that.