I've been working with rails for a long time, so it doesn't feel like magic to me, but I can understand that concern. I highly discourage metaprogramming unless there is an obvious win. Rails/Ruby, especially in the beginning really leaned in hard on metaprogramming, but I've seen that be less of a trend with ruby. From my perspective, the metaprogramming trend has been going down. Ruby makes writing DSLs easy, and that can appear as magical too. In general, if a rails project follows convention it is easy to find what you're looking for.

I don't understand what you mean the global namespacing. Ruby has namespacing, and adding to the global namespace should be avoided except for adding new class definitions. Maybe you're referring to inheritance, and how much stuff can sneak into the current scope from class inheritance.