Walled Gardens
Projects are liable to turn into walled gardens. A walled garden is part of a wiki which is separate from the rest of the wiki, in the sense that it has very few links to or from the rest of the wiki. Walled gardens are bad because contributors to the rest of the wiki are unlikely to notice that they exist, and so they discourage collaboration. Wikis are meant to aim for collaboration: the whole point of a wiki is for when you want people to help you write your pages, and that’s most likely to happen if you don’t put them into a separate project. (Someone please help me explain this better.)
