[[ The short of it is: It’s not true. Kurenai’s genjutsu is vastly inferior to Itachi’s. Itachi is a natural Dojutsu user, and Itachi has also been extensively training his Genjutsu since he awakened his Sharingan. Kurenai lost the genjutsu battle badly to Itachi. This is canon, it literally happens on the pages.

The slightly longer version with the hot take is: Trying to argue that the fight wasn’t fair because Itachi is an Uchiha is pointless. So we have to dumb Itachi down and baby Kurenai so she feels good about herself? She was fighting an S-rank criminal and she knows he’s an Uchiha. She put out her very best and it wasn’t enough. Don’t baby Kurenai, they’re just on two different levels. Fights with good guys V bad guys aren’t supposed to be fair, realistic fights aren’t always fair anyways. Kakashi lost to Itachi too, are we going to argue that Kakashi is written badly/written unfairly/got shafted, etc?

The part of it that’s me going on a tangent is: We honestly need to stop acting as if a female character is “written badly” or “poorly” when she doesn’t have canon-shattering strength and abilities. All this does is create a ridiculous expectation that women characters are only defined by their strength and abilities. She’s a well respected and powerful Jonin on her own, she trained a team of skilled trackers, and she raised Mirai with the help of her team and stayed a single mother, and she overcame a Father who was misogynistic. 

Kurenai is a great character, and the people who try to argue her genjutsu is or “should be” just as powerful as Itachi’s are insanely jealous of Itachi and are overlooking the rest of her character. ]]