Lol Dude Naruto used Shadow CLones to counter Shinra Tensei.Ok, I agreed that destraction an attack brings doesnt end the ight. but If itachi and Pain are in the Chuninarena, thirty feet from each other and Pain uses the giant form of shinraa tensei, Itachi is digged under tons of damaged houses and earth, blown away. Maybe alive maybe dead. Even using Susanoo wont help cause susanoo just block direct hits. If youre blown away by that godtechnique (just the technique is godlike, not the user. Shinraa tensei blows anything exept kyuubi or maybe other tailed beasts away, without any hope of the victims to resist.
You got that much speed, that the landings on the ground are very dangerous. susanoo does not protect from hard hits that smack you away.
The weakness is that you are just protected for so long, you still have your feet on the floor.
Itachi's Susano'o would of course keep him protected from Pain's ultimate Shinra Tensei It may blow him back some. Besides if Nagato uses his Ultimate Shinra tensei that he used to destroy Konoha with, HE will be powerless afterwards, and would be put in a position that he can easily be killed in. Or sealed.