Why is unit testing in supporting complex systems a challenging task?

Unit testing of complex systems is difficult for several reasons:

• High code coupling: Components are tightly intertwined, making it difficult to isolate a single unit for testing.
• Dependencies: Units often depend on other units, external services, databases, and environments, requiring mocks, stubs, or fakes.
• Unpredictable behavior: Complex business logic and interactions with external systems can lead to non-obvious states and behaviors.
• Unavailability of external dependencies: During development or in CI/CD environments, external services may be unavailable or unstable.
• Architecture complexity: Multi-layered or microservices architecture increases the number of units and their interconnections.
• Inherited code: Lack of tests, poor structure, and tangled logic in legacy code complicate writing new tests and refactoring.
• Vague requirements: Absence of formalized documentation and incomplete knowledge about system behavior hinder defining expected test outcomes.
• System scale: A large number of units require significant effort for writing, maintaining, and executing tests.
• Asynchrony and multithreading: Testing parallel and asynchronous operations requires specific approaches and can lead to flaky tests.
• Domain-specific features: Complex subject areas with many rules and exceptions require deep understanding for writing comprehensive tests.