A type checker is only going to add limited value if you don't put the effort in yourself. If everything string-like is just a string, and if data is not parsed into types that maintain invariants, then little is being constrained and there is little to "check". It becomes increasingly difficult the more sophisticated the type system is, but in some statically typed languages like Coq, clever programmers can literally prove the correctness of their program using the type system. Whereas a unit test can only prove the presence of bugs, not their absence.