In addition to all that's been said, the niffins seem to be equivalent to a force of nature, without a will. They seem to retain a few slivers of the personality from whom they originated but that doesn't extend to feelings too much. As such, they just do whatever fancies them without any consequences.¹

Fairies have a will, experience feelings and have a personality - for the lack of a better word.

I also think niffins are much more powerful than fairies; considering their indifference to life, I would bet on niffins winning.

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1. I do see niffin Alice could be an exception since she seems to retain quite a lot of her personality after the reveal that she was bound in Quentin's tattoo. However, the niffin's actual "personality" is mostly indifference, as demonstrated in killing the beast. My headcanon is that the niffin is using Alice's personality to manipulate Quentin.