IMO..
weak against Genjutsu should rather be an individual disadvantage rather than be a clan weakness.
See, Genjutsu is something which can be easily be broken through the release technique
(Kai) . The thing is, the more you train, the better you get in resisting genjutsu. Also, it can be broken by applying a chakra surge around you by your teammate/partner.. one more method is causing pain to your own self to escape genjutsu.... All these can be achieved through training+ experience .... and if there will be any KKG similar to sharingan, breaking genjutsu will be easier for that clan (then again, experience req.)
so basically.... genjutsu as a weakness or strength should be variable from person to person on the basis of their training and experience.
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