I'm on a large codebase that transitioned from Javascript to Flow a few years ago (which is very similar to TypeScript, though nowadays I'd recommend TypeScript over it). Before we started using Flow, big refactors were extremely painful. It was practically guaranteed that we'd have to spend a week manually double-checking things, and then spend at least a few days finding and fixing bugs after deploying the refactor to users. Therefore we avoided refactoring at all costs. With Flow, we suddenly can refactor something, find the loose ends with Flow immediately, be sure about it, and be done with it. The difference between now and then is night and day. We can get things done so much faster now. The article author's experience is completely alien to me.