A more accurate statement would be that the NEA of 1976 has never been challenged in Federal Court. The Youngstown decision, when the Supreme Court struck down23o Truman's emergency action in a Steel Strike during the Korean War happened before the enactment of the NEA.
I don’t think either party wants to challenge the act in court. It was passed after a history of court rulings on emergency powers and was intended to remedy the court concerns. It also seem written to eliminate a role for the judiciary putting the onus on congress to check the president’s power. You can;’t run country with an emergency by the fiat of 1 or a small group of judges, who have no army or intelligence gathering apparatus.