Yeah, what I have read tracks about not having nearly the stuff Darvish has. On the other hand, Darvish might be the best "stuff" pitcher in the entire league - so I am not sure whether "not as good as Darvish" is really saying much. The commentary I have read in that direction make it much more about what Darvish has so much as Tanaka missing anything. There seems to be some consensus that his splitter is plus, the way you expect from most of the successful NPB guys. Certainly it seems like the control is there, and the radar guns have the fastball at least in the 89-92 sort of range. If a comp is prime Kuroda, that is not a bad place to be. Not a true ace, but nothing to sneeze at.
For me the red flag is his missing a month last season. Given how hard these guys are worked from an early age in Japan, there is some risk of whether that is a longer term thing. At the same time, there isn't a FA starter out there without major red flags. Just a matter of what you are willing to live with.