The problem is that there's no way to automatically check any of that. If the code is small and doesn't change or doesn't have to last for too long, that's OK.

The problem is that there's no way to automatically check any of that. If the code is small and doesn't change or doesn't have to last for too long, that's OK. But when it's changed over the years, it's pretty much impossible to guarantee that the unsafe parts are kept small and contained, especially as programmers leave and new ones come in.