Sorry, I didn't mean to imply everything can be well tested, but where it can tests make sense even if they don't cover everything (vs. the attitude that testing is pointless unless you can cover all relevant code paths).

Also, I don't think there's as much value in unit tests than other more holistic types of testing, i.e. functional/integration tests.

TBH I read your linked article and still don't quite know what a test harness (in that context) is and in what way it's better and differs exactly in the concrete. Guess I'll have to read up on it elsewhere.