Sort of related, flails and axes are generally useful for the Ancient dead’s frontline troops. They have shield wall every turn, have infinite stamina and maybe even have shield expert
They do have Shield Expert and Pathfinder, sometimes I see them use Rotation. I only use flails against Auxiliaries and maybe Legionaires because their damage is somewhat unreliable, especially against armored troops. They really work well if you get a head-shot with a hard hitting weapon first which destroys the head-armor.
The 50% critical damage from head-hits only count towards HP. The 50% additional dmg from 1-h axes though counts for both armor and HP. Head-hits are quite strong but I think specializing with Headhunter only really makes sense on Crossbow users, maybe Archers, 1-h axe users (especially axe duelist) and maybe flail users.
The idea behind the Flail is that you can use your normal normal hits and probably get a headshot, then finish them off with the 2nd skill.
It also works for 2-handed weapon users as each hit from a split/swing etc will add a stack of head hunter. I guess the main question is if you can find the spare perk points, really.