DarrkPhoenix: I agree on the ballistas and centaurs, but I'm not sure if I'd classify them as better than catapults for two reasons- range and armor piercing. Catapults have a great range to begin with, which can also be upgraded. While horse archers can compensate for this by closing the distance to their target, this can put them in range of enemy units (both ranged and fast melee), while catapults can stay safely out of range while pelting things on the other side of the battlefield. As for armor piercing, for units with a low ranged defense double-shot from a horse archer with a couple of good medals can do quite a bit of damage, but once you start dealing with units with higher defense that damage can get reduced very quickly. Between armor piercing and the high base damage it's been my experience that catapults can still deal good damage to units that even high level horse archers are only doing minimal damage to.
Ranged units in Eador are at support position, you can't effectively play with them. There are too many possibilities to take damage during battle, like fire ball from mage, or fast manticore with 5 speed, elf+fairy, and so on. Damage sticks to them. They heal very slowly, they get penalty for <50%hp somehow, and their effectiveness capped at ~20 damage/round, maybe 40 with astral energy.
There are much more useful things for T3 slot. Cleric - for revive, dispel and heal across the battlefield. With cleric in your army you can put horse archers under fire. Minotaur/Executioner for dealing with tons of meat, round attack does wonders. Even knights and griffins can be used more strategically than catapult (as bait for slow units). Mage opens so many possibilities with enslavin mind (like sacrificing enslaved unit, or getting roots after counterattack)
Catapult is simply not worth T3 slot in any decent army. Just look at T2 units in attachment - I can put any of them in the middle of battlefield, and they would deal more damage than catapult due to counterattacks. Thug is especially scary with his Crushing blow.
As for horse archers - due to mechanic of Charge they can deal damage even to ghosts, damage from Charge is added to total damage, so on practice it can be the only damage dealt.
gtg, maybe I would add something later.