There´s no invisibility at all at this game.
Wraith ability you´re like a sparkling ball of o light, that can be chased easily.
Mirage, you don´t get invisible, you get a camouflage, like the movie predator.
You can be "seen" because you´re blurred, it´s useful if people are at med to long range, if they´re close to you, they are going to notice, unless they´re getting shot at, grenades and stuff like that.
It´s a distraction, well played in the right circumstances it´s very useful, like every other ultimate at the game, it´s not a free invisibility ambush ult.