As new ways to wage war and inflict mass slaughter are embraced by armies all across the world, so too do the ways of the horse and sword change. In the place of the heavier Demi-Lancers arises a new breed of soldier: the Cuirassier. He retains most of his stats, but differs from the Demi-Lancer with several regards: it has more attack, better hitpoints, a better rate of attack and a slightly better move and production speed in comparison to his more ancient forbears, offering the French, Mexican, and German factions a powerful shock force for the mid-game in Rise of the Moderns. The problem here however is that once you obtain them, Cuirassiers usually cannot be upgraded further.
In short, the Cuirassier although costly and slow for a cavalry unit, is still of use against a variety of targets, but are also somewhat vulnerable to ranged cavalry units and dedicated anti-cavalry units, such as Grenadiers and/or Flying Artillery barrages. The best use thus for your Cuirassiers is to flank enemy infantry and artillery where possible, or to use them in a more defensive manner in protecting your flanks or defending artillery from enemy melee cavalry, particularly from attacks by Hussars or light cavalry.