> But Ruby by itself has a major challenge in that it lacks any kind of built-in mechanism for expressing variable and method type signatures. Every one of us had managed large Ruby codebases in the past and wanted to avoid the quagmire of uncertainty around what the types of anything are supposed to be

Ruby has a lot of ways to express the type of information you're passing around. The problem is when you don't validate things coming from users, APIs, and the database, or if you muddy you state in some overburdened god-object. I think the problem is that its so easy to write Ruby code and it less you do almost anything you want, so it is very easy to make a mess. Types may or may not help.

I've worked in some big Rails/Ruby code bases and usually any mess was the result of someone who didn't know the language writing a bunch of code alone with little to no code review or guidance.