The second biggest weakness of a one-handed backhand is its vulnerability to being late. The proper contact point for a one-hander is farther forward than for any other groundstroke. You can often make your opponent late by hitting a ball with extra speed, obviously enough, but extra topspin or extra slice can cause the same effect.
Topspin will make the ball slow down less when it bounces, thus getting to your opponent earlier than a flat ball would, given the same air speed. If your opponent has not read the spin properly, he will be surprised by how quickly the ball arrives.
Slice has the opposite effect on the ball's bounce: it makes the ball slow down more. This, in itself, would tend to make your opponent swing too early, but if you hit slices deep and fairly fast, you can also make your opponent swing late. Slice makes a ball carry farther through the air, so an opponent who fails to read the spin properly will expect the ball to land shorter than it does. When the ball lands unexpectedly deep, he will often be unable to meet it far enough in front of himself to hit it well.
One-handers are less vulnerable to being jammed or stretched out wide than two-handers, but they are far from impervious to these attacks. Try jamming and stretching all of your opponents on both their backhands and their forehands.