I think that what happens is that the aerodynamic effects from single prop setups become more and more noticeable as the air speed of the plane is reduced.
This is due to several causes. Firstly, the spiral airflow from a single prop is much stronger at low airspeeds with high power loading, because the air passing through the prop disk is accelerating much more than it is when the plane is flying fast, and the motor is also supplying maximum torque into the prop, which then has to react out against the fuselage, and both of these things are changing as the speed of the plane changes, resulting in more workload for the pilot to keep the plane flying straight.
What a Contra Drive does is straighten out the spiral airflow and cancel out about 90% of the reaction torque that has to go into making the props turn, which makes the plane appear to be flying in a much more predictable way at slow speeds.
Brenner ...