According to the lead developer at Twitter, Ruby doesn't scale easily. While I agree there are issues with scaling in Ruby, it can be done. The issues are just different. Where I think people get in trouble is to depend too much on the things that make Ruby so productive. As you scale, you need to be aware that simple line of Ruby code can result in single threaded execution or several database calls. To scale, code may need to be more complex. It's the same with any platform that hides complexity.