I see some of Rails' problems — particularly around performance — as symptoms of Ruby's design. Ruby isn't all that performant to begin with, which coupled with the metaprogramming and magic the language encourages can result in some very slow applications. Rails' issues around performance are also a direct result of Ruby's lack of true concurrency caused by the GIL.

In terms of maintainability, I've found that unless you're one of Ruby's "gurus" or know all the best practices around the language, it's far too easy for the average developer to write code that will be difficult to maintain long-term.