The Emancipation Proclamation, while politically and diplomatically brilliant, was morally flawed. To what extent is this statement true?
This is true to a very great extent. The reason for this is that the Emancipation Proclamation did not really free any slaves immediately. It claimed to free the slaves who were in areas that were in rebellion against the US. But it did not free any slaves who were in areas not under rebellion. This meant, for example, that the slaves in the four "border states" (states that had slaves but remained in the Union) were not freed. In that sense, it was morally flawed because it was more concerned with political repercussions than with doing the right thing and freeing the slaves who were in Union areas.