Firstly, knife throwing by itself isn't easy as it requires good hand-eye coordination. To add on, Mai usually throws multiple knives at once, which adds on to the difficultly of all of them reaching their target.
She manages to shoot her knife through FALLING RUBBLE, and it still reaches its target.
Mai often uses her knives to disarm her opponents, cutting through their clothing without causing any real harm to them. With this level of accuracy, she can definitely aim for vital organs to end her opponents permanently.
Cutting off tapestries in a straight line, without damaging the wood. Need I say more?
A non-bender, taking out multiple fire benders at once all by herself, without getting injured. Again, nerd I say more?