Well the Senju clan obviously rivaled the Uchiha clan..so they must of had some secret to resist there genjutsu having knowledge of it.
The only thing that doesn't make this plausible is that Madara was the first Uchiha to awaken the Mangeyko Sharingan which would obviously have a stronger genjutsu (Tsukuyomi) if Madara has it that is and considering Hashirama fodderized Madara every step of the way is weird so yes the Senju probably have some resistance technique to genjutsu or being senju just makes you that badass *_*
The only thing that doesn't make this plausible is that Madara was the first Uchiha to awaken the Mangeyko Sharingan which would obviously have a stronger genjutsu (Tsukuyomi) if Madara has it that is and considering Hashirama fodderized Madara every step of the way is weird so yes the Senju probably have some resistance technique to genjutsu or being senju just makes you that badass *_*