He's been overly passionate (and aggressive?) on trying to make sure he's right. He's shouting it from the rooftops. He keeps repeating it's all a bubble. All good and well, and it very well might be, BUT that in itself doesn't give him credibility. You can say the same thing about the stock market, and once every economic cycle when we see a major correction, you'll be right. A broken clock is also right twice a day.
At the same time, economists usually aren't very wealthy people.. maybe they are overly sceptical of new developments.
That's a risible critique of expertise with little applicability to Nouriel Roubini in particular. Last time I checked, Nouriel was living in a $5.5 million penthouse in the East Village, and before that he was in Tribeca.