Contradiction with Matthew 28:19
Joel 3:1 speaks about gathering all nations for a future event, whereas Matthew 28:19 commands spreading the gospel to all nations, not gathering them.
Matthew 28:19: Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: [teach...: or, make disciples, or, Christians of all nations]
Contradiction with Acts 10:34-35
Joel 3:1 focuses on God's specific judgment on nations, while Acts 10:34-35 emphasizes God shows no partiality and accepts anyone who fears Him and does what is right.
Acts 10:34-35: Then Peter opened [his] mouth, and said, Of a truth I perceive that God is no respecter of persons:
Contradiction with Galatians 3:28
Joel 3:1 mentions the gathering and judgment of nations which implies division; Galatians 3:28 promotes unity and abolishment of divisions among people.
Galatians 3:28: There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female: for ye are all one in Christ Jesus.
Contradiction with Romans 14:10-12
Joel 3:1 refers to a future national judgment, while Romans 14:10-12 focuses on individual accountability before God.
Romans 14:10-12: But why dost thou judge thy brother? or why dost thou set at nought thy brother? for we shall all stand before the judgment seat of Christ.
Contradiction with John 3:17
Joel 3:1 involves the notion of judgment, yet John 3:17 states that God sent His Son not to condemn the world, but to save it.
John 3:17: For God sent not his Son into the world to condemn the world; but that the world through him might be saved.