Unpopular opinion - Coustilliers would fit better in the Sicilian civilization

Compared to both Coustilliers in Burgundian civ and Serjeants in Sicilian civ

While Sicilian have good siege options and Serjeants, none of their bonuses favour either infantry or siege. The bonus damage resist is mainly a cavalry bonus since cavalry can't be overpowered by brute force, they need a proper counter unit.

For Sicilians, it is Cavalry all game, unless you research First Crusade and go for Serjeants - the transitions are awkward.

For Burgundians, they have +2 attack on Knights (basically Cavalier - Bloodlines) but Castle in early castle age is a big investment. In Imperial Age, when castles are much viable, Burgundians have access to Paladins immediately and much cheaply. Hence the Coustilliers which were seen here and there in Castle Age, become outdated and useless in Imperial Age.

While the charge attack of Elite Coustillier is good enough to one shot villagers, Burgundian Paladins have more HP, more attack, more pierce armor. The charge attack of Coustilliers doesn't work against buildings, which is an important strength of Heavy Cavalry in late game.