Scorpion and Noob are more in control of their actions and thoughts. And got cool revenant powers.

But wouldn't it be more useful for Wuan Chi to make the same with the others to fight Raiden and Co

Scorpion and Noob have more willpower and can do w.e to a certain degree

I'm pretty sure they've made it a point to note that Scorpion specifically is NOT a revenant, and is in fact a Wraith.

I'm not exactly positive what degree of control Quan Chi has over any of these characters. In the old timeline his undead guys like Scorpion and Noob seemed to retain free will. Scorpion still seems to have his to at least some degree in the New timeline. But the New revenants seem to be slaves.

It also seems like a plot hole in the old story that Mileena died and became one of his/Shinnok's servants, but suffered no changes or negative side effects whatsoever, But I assume NRS is just going with the, "Ignore that" plan for that one.

As stated above, Scorpion and Noob are wraith, wroth spirits that retain their human body and free will.
The Revenants are smart and non-cannibal zombies, but they are mentally enslaved.
They move quicker and can speak proper english than zombies.

But MK9's Noob ending said he was a Revenant.

But you clearly see the difference between Noob and the MKX characters

The difference is NRS has horrible continuity.

Scorpion is a specter, not a wraith. Noob is a wraith.

Just saying. Probably the same thing, different name, but whateva'.