The problems with a NL interface is that it assumes problem domain knowledge. That is to say that the user is normally assumed to know the vocabulary and semantics of the tasks they are undertaking. For example an application for use in a financial institution is unlikely to be usable by someone who has walked in from the street.
NL interfaces can require lengthy confirmation and clarification dialogue to take place due to the inherent distrust of the capabilities of such systems.
Any enhancements made to a NL interface are invisible to the end user. In a menu interface, as a comparison, when new functionality is added to an application it is common for a new menu item to be added to access this functionality. However, to a user of a speech interface, these upgrades occur in the background and go unnoticed without explicit information given to the users via memo or similar message.