Power means Chakra, which is the source of the techniques. Halved power doesn't result in an inability to use techniques which he is already capable of to use.Kabuto said power not chakra. Power can be related to strength, ninjutsu, taijutsu and everything. Obviously if all yours skills are halved, not surprising he won't be capable to use such a complex jutsu like Jinton
Does that change the fact, that Itachi used Tsukuyomi, endured it being broken, set on fire a huge area with Amaterasu, deleted it once and used Susanoo for several minutes, along with ordinary Genjutsu, Ninjutsu, all in one fight? No, it doesn't change that fact, it happened.As for Itachi, even if he wasn't at his limit, he was close to it because we rarely see Itachi turning off his Sharingan. If he did that there, it shows he was having issues especially since once again, he requested a rest
Doesn't change the fact that Sasuke reached his limit even before Itachi. Doesn't change the fact that Sasuke was completely exhausted after the fight against Danzo, as he was exhausted against the other Kage and felt pain in every cell due to the usage of Susanoo. Despite we know now that Naruto felt Indra's presence in Sasuke back then, shortly after his fight with Danzo.I don't know where you got it doesn't. Sasuke and Madara are reincarnations of an entity possessing such a powerful chakra it travels through generations and generations. You can't compare them to Itachi.
Yet, with the EMS, he spammed MS techniques and used Susanoo without any exhaustion. It's all about putting 1 and 1 together.