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There are pros and cons to every language and framework. We’ve already heard alot about the positives of Ruby on Rails, here are some cons. These comments were made by Alex P. a twitter developer.

All the convenience methods and syntactical sugar that
makes Rails such a pleasure for coders ends up being absolutely
punishing, performance-wise. Once you hit a certain threshold of
traffic, either you need to strip out all the costly neat stuff that
Rails does for you (RJS, ActiveRecord, ActiveSupport, etc.) or move
the slow parts of your application out of Rails, or both.It’s also worth mentioning that there shouldn’t be doubt in anybody’s
mind at this point that Ruby itself is slow. It’s great that people
are hard at work on faster implementations of the language, but right
now, it’s tough. If you’re looking to deploy a big web application
and you’re language-agnostic, realize that the same operation in Ruby
will take less time in Python. All of us working on Twitter are big
Ruby fans, but I think it’s worth being frank that this isn’t one of
those relativistic language issues. Ruby is slow.

These comments shouldnt discourage people from using Ruby On Rails, when your as popular as Twitter every language and framework is going to run into problems. However, its always good to stay informed and know about the issues other developers are facing.
Read the whole interview via radicalbehavior