Maybe I am just stupid when it comes to reading Ruby/Rails or maybe that codebase was uniquely awful, but it was ~impossible to figure out where things were defined or how data moved through the system. A huge ball of mutable state that was defined at runtime.
When people say "I love writing Ruby" what I hear is "I love writing greenfield Ruby". Everybody loves writing greenfield code! The difference between greenfield and brownfield Ruby is stark, in my experience.
And to be clear I do not hate Ruby. It got me into the industry, it taught me a lot, it just optimizes for a set of values that I don't happen to share anymore, which is fine.