Nouriel Roubini has a well known hate for Bitcoin, which probably originates from the fact that it escapes his alleged "expertise": he hasn't foreseen its popularity, and rise in value. Now he hates it, because this past mistake threatens his status as financial guru, "Dr Doom".

EDIT: I wouldn't take anything he says about it seriously, he's basically clueless on the topic.

EDIT2: In free markets value of something, e.g. Bitcoin is not decided by some panel of experts, like professor Roubini, but by people buying and selling it. If people want to pay $45k for a Bitcoin then that's it's true value. Roubini's claims that "fundamental value of Bitcoin is zero" is riddiculous. Bitcoins are collectibles, same as, say, baseball cards, and baseball cards can go for some insane prices despite having zero practial usefullness.