That flail is certainly the worst of the lot. Terrible impractical weapon with scant evidence of it ever being used by anyone.
There's a two-handed version of the flail that is basically an agricultural threshing tool with spikes stuck into the head, which is much more plausible as a peasant's weapon (very similar development process as the nunchaku, also a highly overrated weapon). However most weapons that feature a flexible rope/chain part in their design all suffer from the same drawbacks: difficult to control, and limited striking power.
The two-handed version of the flail is definitely real and a reasonably formidable weapon. I study at a Spanish HEMA school and we have one source—Mendoza—which basically says it should be used in the same way as the montante (the Spanish greatsword), except without thrusts. Like the montante, its best value is when against multiple opponents, such as doing crowd control or escorting someone. Mendoza also describes his flail as being made of three separate chains with individual weights on them, rather than one large thrasher-like tool as shown in the headline artwork on the Wikipedia page.
The only time a fighter can be assumed to win with even 2 to 1 odds is with a significant material and training advantage. And they'd still be better off with a weapon that isn't a gimmick.