If your system maps (perfectly or partially) to the Test Pyramid, you might find test duplication between service tests (API tests) and unit tests (tests of units that constitute API implementation).
API Test Base intends to make service tests — in the form of integration unit tests — fast, reliable, and cheap to modify, helping make many unit tests unnecessary. Refer to Note 2 of the Test Pyramid article. Citing the key part: If my high level tests are fast, reliable, and cheap to modify - then lower-level tests aren’t needed.
For a complicated internal component, service tests might not be able to cover it well. In this case, unit tests are still needed.