I have Kushina 5* almost full BT (only her healing passive is on +1) and yet EG Kakuzu BT usualy beats me easily. Kushina has her downsides. She can't trigger her own chase. She must be exposed to lower her long mystery cooldown (and, unlike Kazekage Gaara, she does not heal per every hit recieved). And her standard is single target only, which means she will attack her own teammate if under Chaos.

In comparison EG Kakuzu BT has multiple target standard (which means Chaos on him is just a different version of Immobile), 0 chakra no cooldown unmissable interrupt mystery (which pretty much guarantee he denies one mystery per round of his choice) and provides extra standard + heal + debuff cleanse to 2 Akatsuki units (which likely includes himself as I yet to see team with him and more than one additional Akatsuki ninja). His only weakness would be lack of debuff immunity, but that he can get from others and even without it Chaos and Immobile are the only debuffs that hurt him, everything else he will just remove before first action.

Also, if someone just got her from Myoboku then there is a hell of a wait before getting her to 4*, she is gone from events except Ramen.

I can only agree that Summer Kurenai is not the best counter, but not because of a ways to remove that Immobile (those can be dealt with other ways), but because her mystery is bugged and ninjas with natural debuff immunity tend to magically free themselves from it at the start of round 2. Still, Summer Kurenai is good, lost count how many times her Chaos causing passive screwed up my opponents.