Can a scythe be an effective fighting weapon?

No. We do have a scythe tag on the subject that might make the tag cloud with this post if you want more detail. You can use one in an emergency when you literally can’t get anything better. If we’re limiting ourselves to farm implements, a pitch fork or shovel is a more effective weapon. You don’t want to intentionally take those into battle either, but you’re better off with them, than a scythe.