Darvish wasn't a sure thing coming over either. While everyone knew he was as filthy as they get, it was always noted that he doesn't have the best command, he got alot of favourable calls in Japan, and there were concerns over his ability to consistently throw strikes.
Obviously to this point, Darvish has been able to compensate for mediocre command with his fantastic stuff, and has emerged as one of the best pitchers in baseball.
Tanaka admittedly has a different question, and that question is in regards to his fastball. Obviously he has yet to prove it like Darvish has, but coming over, he's the same caliber of prospect as Darvish was. Darvish had ridiculous upside because of his out of this world stuff. Tanaka has ridiculous upside because he can be the starter version of Koji Uehara. Different players (Darvish and Tanaka), but everything points to them being the same caliber of prospect.