There has been discussion about this more than a decade ago by people like Dan North and Liz Keogh. I think it’s widely accepted that strict TDD can reduce agility when projects face a lot of uncertainty and flux (both at the requirements and implementation levels). I will maintain that functional and integration tests are more effective than low-level unit tests in most cases, because they’re more likely to test things customers care about directly, and are less volatile than implementation-level specifics. But there’s no free lunch, all we’re ever trying to do is get value for our investment of time and reduce what risks we can. Sometimes you’ll work on projects where you build low level capabilities that are very valuable, and the actual requirements vary wildly as stakeholders navigate uncertainty. In those cases you’re glad to have solid foundations even if everything above is quite wobbly. Time, change and uncertainty are part of your domain and you have to reason about them the same as everything else.
> I will maintain that functional and integration tests are more effective than low-level unit tests in most cases
Right, that's pretty much the only advice I've seen that makes sense. The only possible issue is that these tests may have a broader state space so you may not be able to exhaustively test all cases.
Absolutely right. If you’re lucky, those are areas where you can capture the complexity in some sort of policy or calculator class and use property based testing to cover as much as possible - that’s a level of unit testing I’m definitely on board with. Sometimes it’s enough to just trust that your functional tests react appropriately to different _types_ of output from those classes (mocked) without having to drive every possible case (as you might have seen done in tabular test cases). For example I have an app that tests various ways of fetching and visualising data, and one output is via k-means clustering. I test that the right number of clusters gets displayed but I would never test the correctness of the actual clustering at that level. Treat complexity the same way you treat external dependencies, as something to be contained carefully.
