Funny, but I disagree. Rails scales, if you know how... We serve millions of customers each month and have implemented a reliable and stable architecture.
Great question. In my career so far I have deployed Web apps in ASP Classic, PHP, ASP.NET and Ruby on Rails. It appears that RoR can only handle a small fraction of activity compared to the other technologies before maxing out system resources. There are some lighter weight Ruby frameworks out there (Rack, Sinatra) that are probably more appropriate for large-scale back ends. Still, I have not worked with any Web technology that is performant enough to confidently endorse.
