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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.

That's also my experience, but consider how many of the remaining 5% would have led to a regression that would have gotten shipped in production if the test didn't caught it ? In my experience, this number way above zero.

Is that because you're not writing tests while you're adding code and just breaking old tests?