I can look at most of the code in a Django project and imagine a basic implementation sitting behind it. And I can work off of those generalizations. I can't do that for Rails.
That's the point about DSLs and Magic -- you have no idea what could be going on behind the scenes, and it's harder to make a guess about why it might be going wrong.
And it's not really bias; I come from a generalist perspective. I worked in Python for about two years, and it's the same way as Ruby. Any library or language is going to require a lot of learning to really master. I floundered in Python until I broke down and basically read through the API reference front to back. And with regards to what's going on behind the scenes, I found Django no different than Rails. Perhaps a little more obfuscated with the way templates and the auto-admin work, but I didn't have as much interest in investigating it at the time.
But, whatever. YMMV, obviously. :)
Maybe if you're just a casual driver that's fine. What if you happen to be a race driver? What if you need to drive in snow, on ice, on sand, at high altitude? It's suddenly a good idea if you have some idea how the car actually works. What if your boss says to you, "We need you to drive 15% faster with 10% less gas", and you open the hood and there's just a big box there labelled "magic" that can't be opened?
Libraries and frameworks are all about not knowing how they work behind the scenes. That's kind of the point.