Not the damage. Not the reliability. The cooldown. Aery only sticks with the target for two seconds before she begins heading back to you. In melee range, that means a two second cooldown. In 'Pantheon is spamming spears on cooldown' range, that's about four seconds. Four spears, four instances of small animals molesting his target for about 80 damage total.
Now let's compare to Arcane Comet.
So, assuming Panth is throwing spears at someone, the first triggers Arcane Comet. The second kicks the cooldown to 8 seconds. The third kicks the cooldown to 16 seconds. The fourth will lob Arcane Comet number two. Assuming both hit, that's ~60 damage.
*Assuming BOTH HIT that's less DPS than Aery.*
Whoever designed the cooldowns on these abilities did not the math.