I am creator of online programming courses (notably the ones on Khan Academy), and I would not feel comfortable delivering a programming course where the sole form of assessment was a quiz. A quiz format can be used to check on some kinds of knowledge, but a programming course requires coding exercises (like the challenges and projects used on Khan). That being said, for other domains, quiz-style assessment can get you quite far, depending on the sophistication of the quiz software. I like quizzes with multiple answer types, per-answer feedback, spiral assessments, etc.