Their infantry is only there to hold the line, and hence cheaper. I wouldn't mess with the dismounted Royals either. Their archers really aren't great at all, but they can go hand to hand. Rohan does field supreme cavalry at top tiers (and more with AUM), but far, far more importantly, they field extremely solid medium cavalry with a powerful charge, as well as versatile mounted javelins and archers, far more numerously than any other factions. That is your true weapon. Neither Rhun, nor Gondor, nor Harad will tend to use 50% cavalry armies. You can.