The trade off you get for longer reach is shallower wounds and less force. Scythe combat consists of three basic parts: Stabbing, Disarming and Cutting.

Since it was originally used as a harvesting tool, the scythe's primary focus is cutting things off. It's an excellent weapon if you want nothing so much as to kill somebody, since it's most effective attack is almost always fatal. Secondly, unlike the Naginata with it's straight blade, when you use a scythe to pierce an enemy your blow has far more force, since you're swinging instead of thrusting, meaning that between the two a scythe is actually the more effective piercing weapon. Third, the scythe's unique shape makes it ideal for disarming your opponent, combining the long reach of a spear with the hooked blade. 

One scythe in the hands of a well trained man is worth at least three Naginatas. The reason scythe combat has almost died out is because as a weapon it's made for killing, and can never be anything else. Even a sword can be used for sport, but not a scythe.