Why should you not rely solely on test cases or checklists in the testing process?

Only test cases: take a lot of time to create and update, may be redundant for routine or exploratory tasks, not always effective for ad-hoc testing.

Only checklists: insufficient detail for reproducing errors and training new team members, difficulty tracking specific steps of test execution, a less formal approach may lead to missing important checks on critical functionality areas.

Optimal: combine both, using checklists for general checks and exploratory testing, and test cases for critical functions and complex scenarios.