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To me this suggests that there is no clear understanding what your units are supposed to do.

My second theory would be that the unit tests are written by the inexperienced developers while the good developers write the other code.

The tests will involve 5 mocked dependencies just to test a simple if statement. They test the implemention and are a waste of time.

Isolated units that are well tested usually don't need to be updated in my experience so they rarely break.

Tests that have 5 mocked dependencies is usually a signed we done messed up somewhere.