Ok first, Susanoo alone can be penetrated.
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That was in Sasuke's case, and it was still not the final form of Susanoo but even with the full form it still can be penetrated.
Itachi's susanoo is a little different due to his shield. But The Yata Mirror is considered one of the three sacred treasures and is endowed with all nature transformations, it can change its own characteristics depending on the characteristics of the attack it receives, making the technique ineffective. Zetsu claimed that this, combined with the Sword of Totsuka held in its right hand, essentially made Susanoo invincible.
It would seem pretty much the ultimate defense, but is it? The Mirror only adapts to the nature of the attack... but if that attack is just PURE CHAKRA then what? So i think that it still could be penetrated if the attack is powerful enough...
I guess we will just have to wait and see.