Tests are critics, and they criticise different things
Unit tests criticise code. Acceptance tests criticise the product.
A useful way to think about tests: they are critics. And there are two distinct things worth criticising.
Unit tests criticise the code. They ask: does this logic do what it’s supposed to? They’re fast because they stay small. Acceptance tests criticise the product. They ask: does this feature, end to end, do what a user needs? They’re slower because they touch more. You need both, and confusing them is a common, expensive mistake — slow product-level tests written for things a fast unit test should own, or thin unit tests standing in for product confidence they can’t provide.
The acceptance tests are what eventually let you deploy with your eyes closed. We’ll build that out in delivery; for now, hold the distinction.