Incomplete requirements can impact the schedule of the project and delay it. That is why requirements validation is essential. The agile approach is beneficial as requirements are validated at the end of every iteration as the end user gets a functioning prototype to give feedback, and it can incorporate changing requirements at any stage. But I would say more important is that the user is explicit in stating their requirements and the development team understands it completely and the business logic of the customer. Otherwise, the gap would be there and would increase iterations and delay schedule.
Research shows that 71% of software projects fail because of poor requirements management. The Standish CHAOS Report, which surveyed 9,236 IT projects, found that the top three causes of project failure were lack of user input, incomplete requirements or changing requirements. That is why proper documentation of requirements is so critical. Use the Agile project management techniques to help deal with the changes and additions.
IT Leadership Consultant (2013) The impact of incomplete or changing requirements on IT Project Success. Available at: https://www.boardroommetrics.com/blog/the-impact-of-incomplete-or-changing-requirements-on-it-project-success-20131222.htm (Accessed: 4 October 2017)