> Refactoring data-flows that go through multiple modules is a huge hassle. Sometimes, we would spend hours just refactoring data-passing, wrapping and unwrapping and tests surrounding modules, because we needed to pass some additional values.

Same experience. Functional/declarative code is more elegant, but a small change in desired behavior might require completely different structure of the code (or an ugly hack). Meanwhile imperative code changes much less even if it's less elegant.