As you say, it was really WWII, and not the New Deal, that brought the United States out of the Great Depression. To this extent, the New Deal had a major shortcoming. It was insufficient to cause the end of the Great Depression.
If you need to argue that its achievements outweighed that shortcoming, you should argue that the New Deal preserved the US economy and kept it from completely falling apart. Had it not been for the New Deal, you can argue, the economy would have been so completely destroyed that the US might have had a revolution or might otherwise have gotten to be so bad off that WWII would not have saved it.
You can argue, then, that the New Deal's major achievement was to keep the US economy afloat until the war could come along and save it.