One of the best things I ever read in an Avatar fanfic was that benders have no idea how to handle a nonbender who closes the gap. They are so used to dueling from a distance and with the elements. Benders don't have the experience or mentality to fight someone who can disable them in a few moves. So I headcanon that because nonbenders have to outsmart and outmaneuver benders without holding any advantage back, they generally get the better of benders. This is clear on how fast mecha tanks, chi blockers, Mai, and Asami can take down any bender.