Productivity is a different metric from safety. It is more reasonable to argue that haskell allows you to write correct code faster than it is to argue that haskell allows you to write "more correct/safe" code.
That's a good way to think about it. Any code can be made correct (for most use cases) with the right amount of man-hours. It's more a question of how many man-hours that is (and how many of them are needed to ensure that correctness is not lost when the code evolves).
