Hey Florin, thanks for the comment!
While AC should provide the great basis for feature testing, often they are too high level for unit tests. E.g. if your AC is to create an endpoint which POSTs a new blog post for example, you would have multiple classes to create this, and multiple sets of unit tests which will all test small parts of the flow but not the overarching feature. So the AC might be "When I submit a blog post to my endpoint, then I can view the blog post on the website"
The unit tests will be more granular like "Given I submit text to the controller, When I call Post, Then the Add To Database method is called" - and you would only be testing the Controller object here with all other dependencies mocked.
It's hard to explain without a better example but hopefully I can write up some in the near future. Hope that helps!