This description of the devastation of locust swarms is not exaggerated. When the really big swarms come to an agricultural area, they devour everything even to the bark of the trees and the roots in the ground. Usually insecticides will lessen the impact on farms and ranches, but it can happen just like Joel describes. The swarm will come like a the light of dawn spreading across the sky, like a cloud moving inevitably toward them. And only by God's intervention will they be saved.
But Joel is using the locusts as an analogy for the destruction that the invading army will cause before the Lord calls for judgment in the valley of Jehoshaphat. He shows the people of Israel the destruction the locusts cause; then he tells them the of the human army that will pursue them. Israel did suffer the things he cites--the people were sold for slaves, the land was divided among the victors, now there will be a repayment in the valley of decision.
The day of God's final judgment is still coming.The valley of Jehoshaphat will be the scene of God's judgment because of the pain the people of Israel have suffered. The final judgment will include other sins and God's plan does not leave out any one. The people of Israel will not escape God's judgment for they also failed in their obedience, but the land will be restored and those who seek God will find comfort and provision, and Jerusalem will be the city of God.
I have noticed a rash of horror movies depicting the "last days" or the "end of the world." None of them is quite as real or devastating as the description Joel gives, and none of them offers the same hope. Joel's description of the repentance and future blessing on the people who honor and reverence God is nestled inside the judgment. It does not make the blessing less real, but it does emphasize the cost. For Christians, that cost was laid on Jesus.