If you fully spec out your code, it's actually quite close. In a project we did that in, the only type errors we encountered were ones that a static system would not have caught either (due to their being caused by incoming data that did not conform to our type expectations; for instance, deserializing JSON to a specific type).
Without specs, it will assume every type is 'any()', unless it has information to infer something more stringent. For instance, if it sees you add 5 to it somewhere, it will instead assume it is a number. Etc. Even if in practice it actually is a list of some kind (and so that addition of 5 will fail). Which, yes, ain't great. Hence why I said it was a gradual transition; it will catch provable errors (i.e., if you call append on that same variable as above, it will note that there is no type that allows both append, and + an integer, and error), but leave plenty of things uncaught that could have been caught had it known the type in question (via a type spec).
