List some drawbacks of waterfall model?

Although it is a very efficient model, still there are some drawbacks that are listed below:

• It is not so convenient when it comes to complex projects in which demands are not clearly stated.
• As we know it follows a sequential approach, it is not possible to go back to any step which can result in problems sometimes.
• No service is delivered until the last step is executed.
• The testing phase can also only be done when the last step is executed and not consecutively.