Note that the code which evaluates the formulas is owned by a different team, and they have loads of unit tests on it. I'm interested in tests for our code, the part that generates the formulas.
Also note that speed is a non-issue; the integration tests I've written so far are so fast they feel more like unit tests.
For the reasons given above I'm convinced that improving our integration test coverage is the higher priority, at least for now, but I honestly can't see any benefit at all to unit testing this code, which feels like a sign that I might be missing something important.
So, is there any purpose to having unit tests as well as integration tests for this particular kind of code, where the sole objective is to generate "code" in some other language, and nobody cares what "intermediate code" is used to achieve the final result?