I gotta go with nunchuks. Anything you sling around on a chain seems silly to me. It seems like nunchuks are only good weapons against the unarmed. Anyone else has more range, usually something sharp in play and there isn’t a limp chain in the middle reducing the force of your strikes.
I thought the misconception was that the chains were exceptionally long. Like even the length of the chain in the picture is kind of excessive. Also that the balls were particularly heavy. Then again I learned this almost 20 years ago, maybe it was a theory that was crumbling as I learned it.
Edit for my own childish amusement: “Pain is stored in the balls.”
A quick dive back into the research says it’s still debated but generally the rough consensus is peasants having flails would have been super common, as they were a threshing tools. In Eastern Europe and Central Asia they were somewhat present as weapons, in Western and Central Europe they would have been pretty rare (and high to late medieval period) but not unheard of. They would have been the Nunchaku of their era. Like a French knight might think it an odd choice if a peer wielded one.
This article is interesting https://www.mdpi.com/2409-9252/4/1/9