Rails itself is not that difficult to debug after it has been refactored to less magic with 3.X. What's difficult to debug is all the plugins written in brittle-dsl-with-lots-of-magic style
That's not necessarily true. I've found more edge cases and odd behaviour from Rails in a project that's always been 3.0+ and never used 2-style conventions than I ever found in, say, the .NET libraries.
"Magic" is definitely a problem when it comes to debugging, but eliminating magic isn't a silver bullet for making code easier to reason about, particularly when there's errors. Rails still, almost without exception, throws obscure errors that give no indication to what actually may have gone wrong, and there's still quite a lot of magic left that can behave in odd ways in ActiveRecord.
This is probably true, rails is more 'community-developed' then the aforementioned plugins. I'd say the average developer is more inclined to open and debug these plugins than rails itself. Which is what the grandparent comment is referring to (plugins and apps used interchangeably).
