As can be currently seen in another thread, certain WNFFers think that not even Albert Pujols is a sure thing. 
When rating prospects, there is the concept of the can't miss or sure thing. Guys like Strasburg and Harper become considered sure things before they even make their big-league debuts. You have to look at prospects and rookies and make educated guesses. And in that context, it is very reasonable to label JZ about as sure thing as you can get, because you have to base expectations on legitimate observations and experience. It is reasonable to think JZ will pan out successfully. Still, nobody really knows for sure whether a guy can deliver up to where he projects, but that is why the qualifier of "as you can get" is there.
I don't see how pointing out that JZ's lack of actual track record of consistent big league performance can be put in the same category with any established veteran, much less a Pujols. When guys have track records, you know what you can expect. People knew what they were getting with Adam Dunn.