I don't understand by what metric you view Harris as more credible and deeper-digging. I would accept that Dennett is deeper-digging because he is a philosopher; deeper digging is the name of the game, and he does indeed dig.

But my read is that while Harris, Dawkins, and Hitchens each have their own distinct style, they all operate at a roughly similar level of depth, which is to say shallow-to-middling; middling in the areas they enjoy the most, and shallow elsewhere. (It is a rather sad state of affairs that even a shallow investigation of most organized religions reveals immense problems--you mostly don't need to go deeper if you merely want to conclude that religious doctrine has elements which are misguided.)

Specifically regarding Harris, two areas where he seems profoundly superficial are first, his treatment of getting an is from an ought, which half ignores the problem and half is an appeal to "but it's so obvious"; second, his treatment of the nature of the relationship between religious dogma and terrorism, where he has repeatedly been challenged by people like Scott Atran who have done extensive research on the topic and Harris has every time I've seen him again just stated his position with the justification of, roughly, "the text literally calls for the murder of infidels, so it's obvious".

Now, nobody really has time to be both deep and broad everywhere, and Hitchens, Dawkins, and Harris do all attempt breadth, so to some extent some shallowness can be forgiven. Nonetheless, I at least can't pick out in what way Harris is clearly less shallow overall.