It's also harder to establish checking boundaries in a dynamic language. If you honestly claim that someone both (a) writes pre-/post-condition checks on all functions and (b) compartmentalizes functionality sanely then you've got an exponential blowup in type verification.

So in practice both (a) and (b) are weakened. Usually it's a whole lot of (a) and a little bit of (b). You can reduce the problem by establishing domains, but that limits reusability and requires its own kind of stringent documentation and discipline to handle.