Why can't you fully rely on unit tests?

Unit tests cannot guarantee quality completely because:

• They only test individual, isolated components or functions, ignoring interactions between them.
• They do not verify module integration and system dependencies (database, third-party APIs).
• They do not cover the user interface and user interaction with the system.
• They cannot detect defects related to performance, security, or usability.
• They may pass successfully but not reflect the real system behavior in the production environment if the test environment differs.
• They depend on the quality of code coverage. If the critical path is not covered by tests, defects may remain unnoticed.
• They do not replace other levels of testing such as integration, system, acceptance, and exploratory testing.

For a comprehensive quality assessment, a multi-level testing approach ("Test Automation Pyramid") is necessary, including unit, integration, UI tests, and other testing types.