But in C# for example, if the system was not designed with dependency injection and everything being an interface it's very hard to build a test harness since you can't mock anything. Which means everything has to be tested manually. (I haven't done any C# in a long time, maybe it's not the case anymore)

So you have to create an interface and classes for every implementations for every type in the system just so I can change its type dynamically. By the time you're done with all the cruft, you forgot what you were about to code.

I'm infinitely more productive in Ruby compared to C#. But I can understand dynamic languages not being welcoming to juniors, since they can code themselves into pitfalls that will bite them later.