The jutsu is hard to detect, but not undetectable. Genjutsu are dangerous because they can be used in alot of different situations. It's not limited to just the battlefield. That's the advantage. Ninjas would be more prepared to face genjutsu on the battlefield than off the battlefield.
Jin and their tail beast experience time different in their world than the real world. Besides, I have already explained how a perfect jin can defeat tsukuyomi without needing their tail beast help.
Here it is again. They have access to at least 2 different chakra types. Naruto and Killer Bee can both access their tail beast chakra and their chakra at the same time. If they are placed in a genjutsu all they would have to do is access the tail beast chakra or whatever chakra is not being control and inject it into the chakra that's being controlled.
Infinite Tsukuyomi was broken. Sage explained that to defeat IT the rinnegan and all 9 tail chakra must be used together -
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