Yeah, the type system helps in that, but for a unit testable product you need factory methods, which produce some valid and invalid cases. If you write simple tests for those it is a type checker on it's own and the type checker is just overhead. If I could choose my next project from two similar implementations where the other would be type checked and the other unit tested with basic datatype factors (at least) I would choose the unit tested one.

I would use typed API's though, because it saves documentation reading time and makes the editor autocomplete to work like magic.