Yeah, that is why it is so effective. Although it could only be used once a decade. Excluding Danzo who had Hashirama's DNA.Yea thats the thing i stated about ka, if no one is there too tell you you're in his genjutsu, you are just about screwed because you think you're doing the actions on your own, thus won't try breaking a genjutsu,
But it would have to be used at the outset of a battle against someone who has summons. Because if it isn't then their summons can at least tell them what's up if not break them out.
Also I wish I knew how KA worked. It can't be eye contact as Danzo had his covered up. Maybe it is just proximity to the person, as when Itachi activated it on himself the crow had to fly much closer to get Itachi in it.
EDIT: to the kid above me we are talking about the third raikage. The one who was an Edo Tensei.