Im pretty sure the Shield blocks everything, I mean the myth was that whatever his sword pierces is sealed in a genjutsu and that his shield can transform into any element to block any attack, as in it becomes water if attacked by fire, etc... Zetsu even spoke that with the sheild and sword Itachi was invincible. And as we have seen the myth around the Sword was true, therefore one can only assume the Shield is as strong as it has been said to be.
On top of which Susano's, without an indestrucable shield are fairly strong. I mean even the 5 Kages couldnt get pass Madara's Susano with it withstanding a super punch from Tsunade who was super powered by Tsuchikage making her super light and fast. The only way for them, the 5 kages AKA strongest ninja in each indiidual country, to bypass Susano was to go underneath it. So with Susano's being as defensive as they are one can only imagine how hard it would be to destroy a Susano who has a shield that is virtually indestructible.
Then again which Naruto are you talking about, because im fairly sure that KCM Naruto super powered by the Sage voodoo magic can pull something out of huge arse that we call plot and find something to deal with Itachi Susano... Just off the top of my head, he makes a clone and attacks itachi from multiple sides, the Sheild can only block one side at a time. "Every Jutsu has a weakness" - Itachi