I have to second what Govan said, but add to it.
With Drupal, unless you really want to get into building your own modules and extensions you are really interacting with an application. Even when you start using CCK, all you are really doing is flipping switches, filling in forms and defining new options for content on the site.
Ruby on Rails is two things, and neither of them bares much similarity to Drupal. You asked "How hard is it really?". To answer that you need to understand what both Ruby and Rails are. Ruby is a programming language designed to make the life of the object purist programmer simpler and more pleasant. So, the first part of how hard is it is simply to answer "how long do you feel it would take you to learn a completely new programming language, like PHP but different".
Rails is an 'opinionated' framework. It's opinionated in that it lays out how a Ruby web project should be structured, as well as providing multiple APIs for everything from database access to web presentation. To answer the "how hard is it" question for Rails then (assuming you know Ruby by this point), you have to answer how much do you need to learn about cacheing, database design, page design, RESTful programming etc etc.
It's not a short journey. you asked if there is an equivalent to CCK for Ruby and Rails which implies to me that at this point your knowledge of programming is somewhat limited. Ruby and Rails interact with the database. CCK lets you define things in a database. Thus, with Ruby and Rails you are effectively bypassing the wonderful dialogs and forms that CCK provides you with and doing the data definition bits yourself, by hand, in code.
From experience, when I've hired experts in another programming language and framework into my Rails teams, it has taken them between 1 and 3 months to get productive, and a further 3 to 6 months for their productivity to start to raise and approach that of the Rails experts on my team.
Thus, in your particular case, I would not recommend a switch away from Drupal to Ruby on Rails.