1) Rails is easy to learn.
Rails has a steep learning curve and that's never going away. From the beginning it was conceived of as an advanced tool for experienced web developers and having a low cost of entry for beginners is not part of the Rails way. Rails makes web development faster by depending on conventions that the developer needs to learn and utilize. There are a lot of these conventions and for a beginner the cost of learning them will be extremely low throughput initially. However, the investment pays off much later down the road when the throughput skyrockets due to familiarity with the framework. Another overlooked thing that makes Rails development hard to learn is that there is a lot more to learn than just Ruby and Rails, there's also things like source code management, databases, front end servers, testing and while these subjects are common to web development in general, you'll still need to learn the "Rails way" to approaching these satellite concerns that make up a typical production application. Brook Riggio created this fantastic chart that shows all the subjects that a Rails dev needs to be familiar with in the excellent article "Why Learning Rails is Hard", one look at the sheer number of items should illustrate why it takes a long time to learn Rails.
2) Rails is equally suited for all types of web apps.
3) Rails means you are working with Ruby and Ruby is slow.
Twitter is still sometimes cited as Ruby on Rails' biggest failure. Twitter was originally developed in Rails and shortly after its creation Twitter went through a phase where rapid growth of the site caused a major implosion of the site's ability to support the growth and the site was often down during a time that was known as the "Fail Whale" era. The solution to fixing Twitter's massive scaling problems was to strip out Rails as the engine of the site and introduce a massive messaging infrastructure made up of a multitude of different technologies. Rails was still a part of the infrastructure but only in a very superficial way serving up the web pages for the Twitter web client. This led some to the idea that Twitter was proof that Rails can't scale and anything "serious" should be done in a different technology. What the Twitter naysayers missed understanding is that the massive messaging behemoth that is Twitter was way outside of what Rails is designed to handle. More importantly however is that the ethos of the Ruby on Rails community is the "right tool for the right job", and that includes using non-Ruby and non-Rails technologies to support whatever solution you are after. If Java makes it go faster, then use Java. There's no reason why it has to be in Ruby, and while you might choose to use a Java search engine rather than a Ruby one chances are that Ruby still can act as an excellent "glue language" connecting your Java search engine to other technologies much like the Twitter messaging infrastructure was laid below Rails displaying pages in the web client. Rails devs should be comfortable using things outside of Ruby. A final comment on the Twitter controversy is that it's not likely Twitter would have ever come into existence without Rails because the low investment needed to try out an idea is precisely how Twitter came to be, and that in itself should be an example of one of Ruby on Rails' greatest successes.