Why? I mean can have control over multiple bodies, but that is still kind of lame.
chikabu tensei can also be stopped very easy from the looks of it.
why would nagato win over Itachi or Sasuke???
It might have to do with the fact he mastered all of the elements and yin yang release by age ten. He had the legendary Rinnegan, once possessed by, yes, S06P himself.He defeated Hanzo, led the infamous elite terroist group akatsuki ( publicly at least), then went on to nuke Konoha to pieces like no one else in the history of the village. In his weakened body, he was still able to revive the majority of said village with one of
his techniques. Partly due to him being an uzumaki, which means he had large chakra reserves.What you call lame, controlling multiple bodies, is basically why he was called invincible since he had no blind spots, could revive said bodies, from far away, plus each of them had unique abilities and together they can defeat the vast majority of people out there- BM Naruto, Madara, Obito and Kabuto are the only favorites.
Your battle scenario:
Itachi nor Sasuke can counter CT on their own, and Nagato can counter everything in their arsenal. He can either sense and repel amaterasu, block it with one of his summons, or simply absorb the flames. He can pull them out of Susano'o with BT or tank everything 'till Sasuke or itachi run out of chakra. Lastly, Tsukuyomi probably will not work against Nagato since the latter's eyes are an evolution of the sharingan, even if it did work it is still close range so he could either keep his distance, abandon eye contact entirely and thus rely on his sensing abilities, or he could end the match with one of his power moves.
Nagato is/was simply one of the most overpowered characters, period.I'm guessing you already knew that.