Unit testing is an obvious choice for something like a parser or algorithm library, for common business logic integration tests provide most of the benefit, and

Unit testing is an obvious choice for something like a parser or algorithm library, for common business logic integration tests provide most of the benefit, and then some. Also, unit tests slow down development as they need to be refactored alongside code, while integration tests can use a more stable interface. This is typically true at smaller scale, one smallish team; when build and test times or team isolation becomes a concern, it's time to rethink your testing strategy.