Mock testing is a useful technique for isolating and testing individual components of a system, but it's not always enough to ensure that the system works as a whole. Here are some reasons why mock testing isn't always enough:

1. Complex interactions: In some systems, components may interact in complex ways that are difficult to simulate using mock objects. In these cases, it may be necessary to perform integration testing to ensure that the system works as expected.

2. Limited coverage: Mock testing can only test the behavior of the components that have been mocked. It may not be possible to test all possible scenarios and edge cases using mock objects alone.

3. Limited scope: Mock testing is typically used to test individual methods or classes in isolation. It may not be sufficient to test the behavior of the entire system, including interactions between components and external systems.

4. Performance issues: Mock testing can be slow and resource-intensive, especially when testing large or complex systems. This can make it impractical to perform comprehensive testing using mock objects alone.

5. Incomplete testing: Mock testing can give a false sense of security, as it only tests the behavior of individual components in isolation. It's important to perform additional testing, such as integration testing and acceptance testing, to ensure that the system works as a whole.

In conclusion, mock testing is a valuable technique for testing individual components of a system, but it's not always enough to ensure that the system works as a whole. It's important to use additional testing techniques, such as integration testing and acceptance testing, to ensure that the system works correctly in all scenarios and under all conditions.