For instructors, a Wikipedia assignment will take more time than a similar traditional assignment. Be sure the instructors understand this at the start! Students gain a lot more skills out of them — writing in a neutral point of view, critical thinking, research, technology — but they need to learn how to accomplish the task, and that will take more time. Since most students will be unfamiliar with wiki code, that adds a layer of complexity. Good instructors will understand that they'll need to spend more class time answering questions about Wikipedia, and assignments will be more difficult to grade because of the collaborative nature of Wikipedia, but they will be willing to put in the time to work with their students because of the benefits to both the students and Wikipedia.
Remember, Wikipedia is likely not the subject the instructor is teaching, and there will be many more demands on their class time (and out-of-class time). Try not to demand too much from the instructors, and acknowledge their expertise with regard to the subject matter and the art of teaching.