Becoming a 4/4 for four mana is more than fair, particularly when said 4/4 can also munch key cards in your opponents graveyard each time it swings in
Its incredibly unexpected and will be sure to garner quite a few Wait, what does that say? reactions from your opponents
Enchantress's Presence, Sterling Grove, Sanctum Weaver, and Eidolon of Blossoms are all incredible payoffs for a deck focused on green enchantments, and they're green enchantments themselves
If your rival had responded to your Unsummon with another damage-dealing spell, your rival could have opted to have that last spell take effect before your Unsummon , giving your creature the deep-six
But for a 2-drop, 3 turns is plenty of time to hit the field and have a significant impact
If everything goes right, and Jace resolves, he will enter the battlefield with 6 starting loyalty, due to Doubling Seasons effect