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Pekora was concerned about the dream's power, after all, dream magic wasn't widely known and wasn't as simplistic as dark and shadow magic in the way that it can be easily countered with another element clashing against it.
Dream magic often presents itself as a pink or black-colored smoke, so it could very well screw with someone's else very easily, and even end up fucking them over with the power of dreams and illusions, we don't want that to happen, right? Due to that, it would be recommended to simply not fall for the dreams at all.
However, it is quite hard to even consider it as a widely spread magic. Dream Magic is hard to wield, incredibly hard, and I bet this guy cannot easily infuse it into other people's mana cores as easily as he did it with darkness and shadows.
Check "The Protagnist's Sister Is Actually the Strongest"!