A friend, who has been a long time Java developer, asked me if there's any truth to the idea that you need to be a better programmer to utilize Ruby. While I wish it were true, I don't believe that to be the case.
The truth is, weak programmers probably wont get themselves in trouble with Ruby because they don't know enough to do anything interesting or dangerous. I've seen plenty of Ruby code that can be written in exactly the same way in Java or C#.
Average programmers are the most likely to cause problems because they can become drunk with their own power and do things that they shouldn't, but can. But, I'm not sure that the situation is any different in a statically typed language. I've seen interesting things like an application that utilized a dependency injection framework to create every object in the application. Clearly someone was overextending a concept and the project suffered because of it.
Good programmers are generally good regardless of the language. They tend to care about things like performance, maintainability, readability, and extendability. If you keep those things in mind, you generally don't get yourself in trouble no matter how much power a language bestows upon you.
You can write good or bad code in any language, and more junior team members slow productivity no matter what tools you give them. Utilizing a less powerful language just increases the chances that they will cause trouble in some other way. The only real way to ensure that a junior team member wont get you in to trouble is to work with them and raise their skill level.