Unit tests tend to be hyper-specific. Even after 20 years of writing unit tests, I find that getting the right balance between not enough testing and too much c

Unit tests tend to be hyper-specific. Even after 20 years of writing unit tests, I find that getting the right balance between not enough testing and too much can still be quite challenging in some cases. For example, some things are a lot easier to test than others. Some things require a lot of setup that is tedious and error prone. In such cases, it's preferable to test at a lower level but it's not always possible to do that and still cover all relevant cases. Those are the cases where unit tests tend to be the most expensive. 100% test coverage may be desirable as a goal, but not at any cost, and tests absolutely do incur costs. So there is a trade-off, and it's worthy of consideration.