The frequency of use and even existence of some types of military flail has been a debating point for some years, This recent article

https://www.mdpi.com/2409-9252/4/1/9

presents quite a bit of evidence on the matter. I don't think it is the definitive discussion but I think it has some useful material.

I've swung left and right on the issue, over time.
Now i take a moderated view, of them being a common improved weapon on foot (there a tool used all over by most people), and an uncommon cavalry weapon (limited use in mostly eastern Europe).
The full metal ones are a rare item, the down sides of a fail in armored combat are many and it benefits few.
(poor lever, readable strikes, limited angles of attack, limited ability to deal with armor. no thrust vs a longer reach and flexible)

The flexible nature and longer reach come in to there own when a flail could be unwound is use, with it being
twirled around like a cowboy lasso and then let out to strike at a distant target or used like a whip.
Great for when attacking over an barrier like the side of war wagon or lashing out from a moving horse.