They are the top AP unit in EB, apart from the lowly slinger :P

Against normal enemies, the falx is of less use than a spear or sword. Peltastai don't wear too much armour, and carry swords, and have more men, making a head-on match with Rhomps seem horribly unbalanced. However, if you set the Rhomps loose on the flank of some Spartans, they will tear them apart. These guys even work against cataphracts.