they get destroyed by anything, their mace has too low damage it does like 20 damage to other heavy infantry, while oaths have better maces and better armor, khuzait spear infantry has super OP maces that 2 shot sergeants, varyags have super one handed axes with 90 DAMAGE, legionaries have a super axe too, combined with improved armor its very usefull against many troops. Sergeants have nothing special, bad mace, bad sword, bad bastard sword that can't be combined with improved armor, they are just not viable in any scenario.