The fundamental purpose of unit tests is not verification. It is sustainable software maintenance. Verification is a side effect. The real value: unit tests enable a codebase to evolve over time without degrading. Tests that verify behavior but are themselves unmaintainable defeat the purpose; tests so complex nobody dares modify them are net negative.
When maintenance is the north star, every testing decision follows: simplicity over cleverness, behavior over implementation, independence over shared infrastructure, readability over DRY.