100% coverage doesn't mean much if you aren't testing that the low-level code is doing the right thing.

For example, imagine you're testing a calculator app. Your integration tests make sure that it never crashes, the UI works, basic math is working, etc, but maybe the sin() function is only accurate to two decimal places.

Edit: I do not mean to imply that you, specifically, are missing things. Rather, it is possible to write tests that have 100% code coverage while missing many possible bugs, and I think those risks are increased without the presence of unit tests.