The scythe adds a small bonus (some 13-15%) to all crop yields, so it’s slightly better than the shovel, but still rather ineffective compared to the more expensive tools. Tool bonuses are cumulative and can be seen on the annual report.
In case of an Angry Mob event, the player will lose all his tools, including any scythes.
Theft means the loss of all scythes (and shovels.)
If Civil War breaks out, there’s a 75% risk that each scythe will be lost or destroyed in the conflict.
With only four tiles for tool placement on the farm, it is recommended to sell the less efficient tools and buy better ones, as soon as they can be afforded.
WikiPedia entry on Scythe:
Scythe Works Without Borders: