It's really hard to get an accurate read on Contreras or compare him to anyone, because he hasn't really put up many innings of stats to look at. All you can go on is his two starts already this year when he shut out the Tigers for 7 innings and allowed 2 runs in 6 innings against the Reds. He is averaging a K per inning which will help you in that department. I would think he will be prone to inconsistency and wildness from time to time, and his performances in relief show that he can be very combustible.
Other than that, there's no real way of knowing and he's very much a crapshoot. One thing that is for sure is that playing for the Yankees, he will have an excellent shot at a win every time out.
Whether he's a worthy addition or not depends on who you have to drop to get him.