> Would also emphasize that while unit testing is often useful for the individual developer working on a piece of code, but as far as ROI, end-to-end integration testing has the most bang for the buck. If you have the full endpoint tested, for instance, from end-to-end, including database writes, your confidence level goes up by an order of magnitude when you have to modify that functionality. If you have to choose between investing in extensive unit testing and extensive end-to-end API or contract tests, always choose the latter.

It's not so clear-cut, unfortunately. Extensive end-to-end tests can be very hard to setup, can be flakey, can take forever to run (especially if you do it on every change), etc.

I agree you should have some layer of automated end-to-end testing (and not enough people do), but in the end, I think you have to work towards making the system as compositional as possible, so you can also test things extensively in isolation - the end-to-end tests then serve to make certain that the individual pieces fit together, but don't hit all the edge cases.