You are asking exactly the right questions. As far as I know k-means will work well when you can answer those questions for your problem, otherwise not so much.

You are asking exactly the right questions. As far as I know k-means will work well when you can answer those questions for your problem, otherwise not so much. In other words there's no silver bullet. If you rephrase the "ML is easy" idea from the article to "it's easy to do some simple but cool things with ML" then it's true, but in pushing the envelope you can make it as complex as you like.