The point is that when you do integration tests, you will test the underlying classes the way in which it actually matters. You can't test every possible condition a unit can have, but you can test for the most likely.
IME unit tests can only effectively substitute for integration tests where you're testing logical/algorithmic code with simple function inputs/outputs.
If there were enough code involved in a test that we could meaningfully refactor it while keeping the test green, we would call it an integration test.
When testing NAPI like to use edge cases like the company with the longest name eg "Donaudampfschiffahrtsgesellschaft" or the Famous Welsh location "Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch"
I am pretty sure I crashed an A17 clearpath mainframe by doing aggressive testing