For impracticality, there are 3 or 4 general issues that have to be dealt with.
Training.
Range.
Purpose
Availability.
An unfamiliar implement or a tool you are not proficient in using, will automatically hamstring you. For any implement, longer range is better. The longer the range, the more cushion for lack of skill or bad luck. If your purpose is to use a blade to exsanguinate a foe or use a sharp point combined with a good range to overcome those with shorter weapons, then if the implement you are using fits the bill then it will be of use.
Then there's availability. None of these matter if you don't have it available on hand. If the scythe is 10 feet away and the opponent is 3 feet away, there's a problem. It's not just a matter of something existing or not. You have to be able to access it to use it. And that includes time as well.
Range is not a matter of skill. Thus a person with a crossbow, even if he is a bad shooter, can kill a knight that has trained for decades with the sword. That is why range is a different criteria than skill/training.
For a farmer using a scythe, the advantages would naturally be his familiarity with the shape, weight, and balance of it plus the fact that it offers additional range than a shorter melee implement.
A farmer would not know what to do with a sword even if he had one. A spear is much easier to handle and use for beginners. A group of spear users can prevent a sword user from getting in range through teamwork. This then uses numbers to offset skill. A spear would be preferable over a scythe, especially for group training. But availability must be considered as well.
EDIT: The chains were so they could weigh down, immobilize, and lock down the sword or sword arm and close in for the kill. That's what the Japanese chain and blades were for, swinging on those centers. It is not a farmer's tool but something specifically designed.