Compared to other similar weapons, there seems to be zero reason to use a flail.
Weapon NameFlailWarhammerCost10gp15gpDamage1d8 bludgeoning1d8 bludgeoningWeight2lb.2lb.Properties-Versatile (1d10)
Both weapons are martial.
I can’t imagine a scenario where a character feels they need to forgo the versatile trait to save 5 gold. Especially classes that can start with any martial weapon they choose.
What advantages does a flail have? Am I missing something?