According to Wikipedia with regard to controversy surrounding his work on Notre Dame:
Viollet-le-Duc's restorations sometimes involved non-historical additions, either to assure the stability of the building, or sometimes simply to maintain the harmony of the design. The
flèche, or spire of Notre-Dame de Paris, which had been constructed in about 1250, was removed in 1786 after it was damaged by the wind. Viollet-le-Duc designed and constructed a new spire, ornamented with statuary, which was taller than the original and modified to resist the weather, but in harmony with the rest of the design. In the 20th century, his flèche was a target for critics.
He was also criticized later for his modifications of the choir of Notre-Dame, which had been rebuilt in the
Louis XIV style during the reign of that king. Viollet-le-Duc took out the old choir, including the altar where Napoleon Bonaparte had been crowned Emperor and replaced them with a gothic altar and decoration which he designed. When he modified the choir, he also constructed new bays with small gothic rose windows modeled on those in the church of
Chars, in the Oise Valley.
[10] Some historians condemned these restorations as non-historical invention. His defenders pointed out that Viollet-le-Duc did not make any decisions on the restoration of Notre-Dame by himself; all of his plans were approved by Prosper Mérimée, the Inspector of Historical Monuments, and by the Commission of historic monuments.
[31]
He was criticized for the abundance of gothic gargoyles, chimeras, fleurons, and pinnacles which he added to Notre-Dame Cathedral. These decorations had existed in the Middle Ages but had largely been removed during the reign of Louis XIV. The last original gargoyles had been taken down in 1813. He modeled the new gargoyles and monsters on examples from other cathedrals of the period.
[10]
He was later criticized also for the stained glass windows he designed and had made for the chapels around the ground level of the cathedral, which feature intricate gothic designs in
grisaille, which allow more light into the church. The contemporary view of the controversy of his restoration is summarized on a descriptive panel near the altar of the cathedral: "The great restoration, carried to fruition by Viollet-le-Duc following the death of Lassus, supplied new radiance to the Cathedral- whatever reservations one might have about the choices that were made. The work of the nineteenth century is now as much a part of the architectural history of Notre-Dame as that undertaken in previous centuries."
[32]
The restoration of ramparts of
Carcassonne was also criticized in the 20th century. His critics pointed out that the pointed caps of the towers he constructed were more typical of northern France, not the region where Carcassonne was located, near the Spanish border. His critics also claimed that Viollet-le-Duc sought a "condition of completeness" which never actually existed at any given time.
[33] The principal counter-argument made by Viollet-le-Duc's defenders was that, without his prompt restorations, many of the buildings that he restored would have been lost, and that he did the best that he could with the knowledge that was then available.