I mildly disagree. componentWillMount() is slightly superior to the constructor, though only for very specific situations[0]. I would agree that for the vast majority of people who use ES6, the difference basically doesn't exist, but there is a real reason to use componentWillMount instead of the constructor.

0: https://github.com/reactjs/react-redux/issues/129

Edit: You might call dispatching in a constructor an anti-pattern (or having any side-effects at all), but quite a few people do it and it can cause very subtle bugs that aren't entirely reproducible.