A unit test is the least useful kind of test. It requires your design to be "easy to unit test" instead of simple, and if you change something and have to rewrite the test you might miss some logic in both pieces.

Plus the tests never break on their own because they're modular, and each time you run a test that was obviously going to pass, you've wasted your time.

As long as you have code coverage, better to have lots of asserts and real-world integration tests.

Integration tests are much slower usually, and you are testing tons of things at the same time. Something breaks (like in that example) and you have no idea of what and why went wrong.

If you unit test properly you are unit testing the business logic, that you have to properly divide and write in a modular fashion. If you want to test a more complex scenario, just add initial conditions or behaviors. If you can't do that or don't know how to do that, then you don't know what your code is doing or your code is bad designed. And that may be the case we read above.

Tests rarely break because they help you not breaking the code and functionalities, and they are so fast and efficient on making you realizing that that you don't feel the pain of it.

I can't imagine any example where "easy to unit test" != simple

in my work with python, easy to unit tests usually makes things a bit harder. You want functional methods , not mega classes with 100s of class variables , and each class method operates on some portion of those class variables. It makes it impossible to truly isolate functionality and test it. While coding though, it is very easy to make a class and throw god knows what into the class variable space and access those variables whenever... However if we have staticmethods , not reliant on any class , just the arguments provided, and it doesnt modify any class state, the test are great. We can change/refactor our models with confidence knowing the results are all the same.

In my opinion, the only thing that is valuable about unit tests is more appropriately captured in form of function, class and module contracts (as in "design by contract"). Unfortunately very few languages are adopting DbC.

Functional tests now, that's another matter. But a lot of TDD dogmatism is centered on unit tests specifically. And that results in a lot of code being written that doesn't actually contribute to the product, and that is there solely that you can chop up the product into tiny bits and unit test them separately. Then on the test side you have tons of mocks etc. I've seen several codebases where test code far exceeded the actual product code in complexity - and that's not a healthy state of affairs.

In more recent times I've seen some growth in interest around contract testing. Unit tests are immensely more useful when paired with contract tests, but unfortunately without them they tend to be more of a hassle. At its essence integrations are a form of a contract, but those suffer their own problems. In rspec you have 'instance_double' which is a form of a contract test as well, but not really sufficient for proper testing IMO. The current state from what I've seen is a little lackluster, but I wouldn't be surprised to see a growth in contract testing libraries for a variety of languages popping up.

I had some tests on my codebase, but eventually only documentation and integration tests remained.

So let's look at a simplified example.

https://bitbucket.org/iopq/fizzbuzz-in-rust

My tests are in the test folder. They are actually superfluous since integration tests test for the same thing.

I cannot break up the program in a way that would unit test a smaller piece of it in more detail. They only tests I can add would be to test the command line driver

For a single person and their one-person code base, you can certainly get away without unit tests.

This is especially if your "integration tests" are testing the same component, and not actually integrating with numerous other components being developed by different teams - or, if the system is so small it can run on a single workstation.

Working in teams on larger systems, the situation is different. Part of the point of unit tests is the "shift left" which allows problems to be discovered early, ideally before code leaves a developer's machine. It reduces the time until bugs are discovered significantly, and reduces the impact of one dev's bugs on other devs on the team.