This is untrue. Structured and unstructured interviews each have an appropriate time to be used. In the initial stage of a project, the as-is process can be unclear so the interviews should be unstructured. Unstructured interviews seek a broad and roughly defined set of information. More open-ended questions and less closed-ended. However it does require the interviewer to come up with newer/probing questions on the fly.
-Structured interviews are better later on in the process, when the interviewer knows a bit more about the basic underlying information about a process. At this time, the interviewer can get more specific information from closed-ended questions.