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.

It covers the subject fairly well.

Trivia: In El Cid in the scene where Heston-stunt-double is fighting on horseback with a flail, if you slow-motion the action, you can see him smack his horse in the head several times.

It is interesting how much the subject is plagued by unclear terminology. That scourge and flail were the same word certainly challenges any interpretation of the art that wants to draw a clear distinction. I also wonder about the identification of artefacts. On the Popular Medievalist site, he doesn’t make it clear why the museum examples are thought to be fake. Similarly, how does one tell the difference between flail heads and weights?