The thing about end-to-end and integration tests is that at some point, your test has to assert something about the code, which requires knowing what the correct output even is. E.g., let's say I've inherited a "micro"service; it has some endpoints. The documentation essentially states that "they take JSON" and "they return JSON" (well, okay, that's at least one test) — that's it!

The next three months are spent learning what anything in the giant input blob even means, and the same for the output blob, and realizing that a certain output in the output comes directly from the sql of `SELECT … NULL as column_name …` and now you're silently wondering if some downstream consumer is even using that.