Can I get an example of what A+ psykers can do?
That's the grade of people like the emperor, Magnus and malcador, meaning they can destroy world, mind control them, teleport them, bring back the dead on a massive scale, stop time and all sorts of similar goodies.
Usually they are incredibly unstable.
Chaos is, if I remember right, the third most dangerous threat (with Necrons and Orks being worse, mostly because I remember it being implied that the Orks might have colomized most of the "uncivilised/unknow" universe, so a unified Waaaagh would be potentially the worst threat imaginable), and second most evil (I vaguely remember the Necron being worse) Correct me if I'm wrong
Chaos are the most evil and one of the greatest threats.
The necrons are the greatest, they are basically soulless undead mechanical skeletons, but if the place of power was created by them, maybe they never got their soul sucked by the star gods and became undead.
They are not really evil, but they are genocidal, and they got the most advanced tech, as in humans can't even understand their technology with thousands of years of research.
The Tyranids were likely attracted by the astronomic on, a giant galactic scaled psychic beacon empowered by the emperor, who is fed thousands of psykers a day to keep him on life support, they are basically adaptable space locust, genocidal, but emotionless as far as we see.
Dark Eldar aren't a major threat on the galactic scaled, they basically got a star system sized city in an alternative dimension, they need to feed on pain to live, and they enjoy it, doing fate's worst than death regularly and engaging in pirate raids to kidnap entire worlds as victims for pleasure and food.
Orks could be extremely dangerous, a biological weapons that can grow incredibly quickly if it has time to ramp up, currently Orks might be the most widespread species in the galaxy, but they are not united, of course, by 40k, a super ork attempting to unit them is rising, and if he did their evolution will accelerate.
Chaos are mostly empowered by their victims, chaos have no true power by itself, they were basically powerless as far as any big power was concerned before the fall of the Eldar.
After the Eldar fell (splitting into multiple relatively minor but still very advanced factions), the heart of their empire was ripped apart by the birth of slannashi, a god they inadvertently brought into existence by their hedonism and decadence.
They basically murdered fucked each other while worshipping a pleasure cult until reality broke, this created the eye of terror, a massive place in space where reality and the warp are one.
Likewise, chaos gods were empowered all over the galaxy, they corrupted psykers, which led to the birth of cults and the summoning of demons, they used the eye of terror as a massive base for their followers and they all around screwed with everyone.
They are very much powered by their victims, our own demonic cults is a good example of their metrology.
They became a solid faction only after the heresy, mostly compiled of humans.
If cultivator power spread, it will rob them of their easiest method to tap the material world, psykers, while also empowering many powerful enemies.
Without enemies to call them into the material world, they are significantly weakened, and the stronger defences could much more easily stomp the cells who do pop up.
Cultivator powers are horrible for them, because it empowers everyone but them, unstable psykers are good for them, because they don't have any problems with corruption and such, you are basically grabbing a big chunk of their own power, and gift it to the mortal races.
This is a massive threat, if they know that should the tide be turned against us, the information will be released, they will need to be much more careful when attempting to get rid of us.