Mūn'ai is a Dojutsu, the only weakness to the bloodline is that it relies on the moon being out but once the moons out, one can cast a world wide Genjutsu on the world, even if the sun begins to rise the genjutsu stay's in effect until the user releases it.
While the Genjutsu is in'a effect; anything that happens does not happen in real life, for example: If you are killed by someone inside the Genjutsu, then when the effect is canceled, you will wake up as if it never happened in the first place. Also once the Genjutsu is active, the user 'If trained well' would only need a sliver of there chakra to maintain the Genjutsu.
It should also be known that the Eight different cycles of the moon cast different Genjutsu.
New moon: Will make the world be cast over with Pitch Black Mist, this will make everyone's scene of detraction Blind to them and make most people go insane or Kill themselves.
Full moon: Will make everyone witness the death of everyone they love; this will have different outcomes; A serial killer is born, A mentally Insane person is born, Mass suicide or Stronger Family Bonds.
(I'm stuck at the moment with this) - (Editing as soon as i come up with more ideas)