Well, he became crippled versing Hanzo, and since then the he used the 6 bodies ... which is basically a long time O.O ~Dem Uzumaki reserves dou XD~
Itachi doesn't have the stamina to hold Susano'o for a long time, his Genjustu can be negated by anyone of the paths disrupting the other ones Chakra flow, Ninjustu wouldn't affect Pein, both have shown to be very proficient in Taijutsu, but like i said, Itachi would not be able to take on all 6 at once, SM Naruto lost -people forget this- its only due to the 9 tails rampage he managed to survive being captured.
Itachi was exhausted by using Amatarasu and Tsukoyoumi in one day. A fight against Nagato/Pein would force him to use them multiple times in one day. Not to mention Amatarasu can be repelled with ST.
Nagatos/Peins Summons.. I really don't need to explain these.
If Itachi were to take out a few of the paths, Pein could easily revive them. Peins CT needed 3 top tier shinobi using there most powerful long ranged attacks to destroy it. Itachi alone would not be able to do it IMO ._.
