Functional programming is one of the oldest programming paradigms. However it isn't used much in the industry compared to more popular paradigms. But it have largely been emphasized in academia.

What's your strongest opinion against functional programming?

The problem is that most common code inherently involves state -- business apps, games, UI, etc. There's no problem with some parts of an app being purely functional; in fact most apps could benefit in at least one area. But forcing the paradigm all over the place feels counter-intuitive.