What is the main disadvantage of using the Waterfall Model?

During the "Requirements stage", the client may discover challenging to conceptualize their needs in terms of useful details. So, there may be a chance to alter their minds once they see the conclusion item which can be troublesome to address if the change in the application needs to be re-engineered to any large extent.

Disadvantages of the Waterfall Model,

1. It does not permit the amendment because it follows a sequence.
   
2. Once the application is in the testing stage, it is exceptionally troublesome to roll back any alter something that was not well-thought-out within the concept organized.
   
3. There's a high amount of risk included and instability.
   
4. It isn't a great model for complex and object-oriented projects.
   
5. It isn't reasonable for long and on-going projects.
   
6. Not appropriate for projects where requirements are at a moderate to high risk of changing.