This is in many ways a matter of opinion or interpretation. There have been at least three major interpretations of Reconstruction by historians.
The first interpretation held the Reconstruction was a failure. It said that Reconstruction was a failure because it gave power to greedy “carpetbaggers” and “scalawags” and to unready ex-slaves. These people were venal and/or incompetent and therefore they did nothing right during Reconstruction.
During the Civil Rights era, this interpretation was called into question. Now, Reconstruction was seen as a “splendid failure.” It was still seen as a failure in that it did not bring about equality for African Americans in the short term. However, it was seen as “splendid” because it was seen as a precursor to the Civil Rights era. It was seen as a time in which African Americans and their white allies worked hard for justice, only to be thwarted by the violence of the KKK and by white supremacy. In this way, it was a short term failure but a long term success.
After the Civil Rights era, some historians have come to see Reconstruction as a failure again. This time, they argue that African Americans failed each other. These historians say that African American elites who got into government failed to try hard to help poorer blacks.
Thus, it is hard to say whether Reconstruction was a failure. There are different perspectives on the question.