There is a Comment back then i Think you didn't read :
The genjutsus differ from one to another, you can't campare a normal genjutsu to a Sharingan genjutsu
The main reason people can't escpae itachi' Gen. Because he Uses either:
*Paralizing Genjutsu : Paralizing the enemy and Making him Unable to Move so he can't even focus on freeing himself or Controlling his chakra before the user get him (like the one used against orochimaru and it takes like an instent so tsunade can't be helped
*Distracting Genjutsu : Uphemeral :The enemy here won't even Know he is under a genjutsu , Like what happened to naruto he was shocked when he saw everybody Turned to Itachi clones and Itachi flying and Stuff , This genjutsu make you able to manipulate The enemy on your free will (making naruto cry , showing him Stuff that doesn't even exist and this is a Strong Type of Genjutsu because a Teammate won't even know you're under a genjutsu ,(he won't see you shocked or paralized) instead , sakura said to naruto that he was running and Using rasengan in the emplty Space , so how is Katsuyu going to know if tsunade is put under a genjutsu When itachi will chose the right moment to attack Tsunade when she is in Chaos ?