The most dangerous magic type in my world is probably EnergyWeaving. Everything in my world is constructed out of these strings of energy called Threads. Time, space, matter, reality itself, every living thing, their basic structure is made up of these Threads. And if a Weaver possesses a reservoir of the universal energy within them, making them essentially 'the same' as these Threads, they're able to manipulate them (they can only be manipulated by something with the same frequency as them, by something the same). And if the manipulator gets creative enough, they can cause some serious damage. An EnergyWeaver has the capability to rewrite the codes of the universe. They can change the laws of physics by rearranging the way these Threads are woven together. My character, for example, is able to create a parallel universe and completely destroy the concept of balance by destroying chaos all together (you may think eternal light would be good, but eternal light is just as extreme as eternal dark). If the Weaver is powerful enough, they could create and destroy entire dimensions.
And why haven't people with this power taken over the world?: Well, only one character in my entire universe really possesses this ability (there's a kinda complicated backstory as to how), and she's terrified of her own abilities. But her abilities were (or will be) abused by another person who uses her to get what they want. So, yes, her magic does cause mass chaos.
Complications: In my world, magic is like a toxin in the brain. The more magic a user has, the worse it is on the mind. It can drive the Weaver to insanity. But not every powerful Weaver grows insane. I think of it like this: imagine a thin shaft. At the bottom of the shaft is something breakable, like an egg. Now drop a rock down it. The rock gets wedged in the shaft and doesn't crush the egg. Now drop another rock. And another and another. Eventually, the weight of these rocks stacked on top of each other is going to cause the force of gravity on the bottommost rock to overcome the force of friction on it. The rock falls and there are two options here. Either the rocks fall and crush the egg, or they line up in such a way that they get wedged in the shaft again before hitting the egg. Now imagine that egg was your sanity and the rocks represented the amount of magic you possessed. If you have less magic, which means less rocks and less weight, it's less likely that the rocks will fall and crush the egg. But sometimes even one rock, if positioned wrong can slip and crush the egg. And sometimes, if positioned just right, 10 rocks can become wedged in the shaft and leave the egg unharmed. It's like a game of chance. (god I hope that makes sense, it makes sense in my mind) But the point is, EnergyWeaving is very powerful magic, which means a higher chance of the rocks slipping and crushing the egg, a higher chance of losing your mind. And it's pretty much guaranteed that your mind will begin to deteriorate with magic of that strength (Sorry, this is so long, if you made it this far, I hope it was sorta interesting to read)