Risks of Ruby
It's just another scripting language.
Bruce Tate on InfoQ: Myth versus reality: Rails is a silver bullet.
People have failed with Rails, and many more will fail. If you apply it without the requisite skills, you'll fail too.
On a similar note, if Java's not your problem, Ruby will not be the answer. Most software development problems are not related to technology. If you're thrashing, Ruby on Rails will only help you thrash faster.