Using a flail to thresh wheat is nothing like attempting to use it against a moving, living target. I've been to SCA events and I've seen several people bean themselves after a bad swing that missed the target or bounced off armor/shield - though those have all been the one-handed ball-and-chain flails.
The flail is, of course, an agricultural implement so I'm wondering why it's listed as a Martial Weapon in the PHB. Perhaps reducing the damage to 1d6.
And how about adding a Great Flail, a two-handed Simple Melee Weapon doing 1d10 bludgeoning, ignores shields, and usable with the Polearm Master feat? Both types of flail would be interesting choices for a wizard with the Peasant Hero background or a cleric of an agricultural deity.
(The Mangual in the video would be a Martial Weapon.)
I would have a hard time believing that someone like me would be able to just pick up a flail and use it at all, much less with proficiency. If I had to fight for my life and I had time to pick up either a club or a flail, i'd pick the club. Heck, I'd pick a sword before I picked a flail. The chain, the spiky ball, the stick... It just seems like a lot to manage.
The idea that peasants "knew how to fight with" agricultural tools is a nonsense. You see peasant armies equipped with them because that is all they had. Proper weapons where expensive and required special skills to manufacture. And frankly, your peasant conscripts are just a meat shield to keep the enemy occupied whilst your proper soldiers do their work, so there is no point on going to the expense of equipping them.
Many of the more weird and wonderful weapons of the medieval period where not produced for battle, they were produced for the tourney, to look cool for spectators.
Exactly. When you learn to use a weapon, the first thing you learn is how not to hurt yourself with it, then how not to hurt allies with it, then how to defend yourself whilst wielding it. How to hit the enemy is the last thing you learn.