How does this artwork elucidate its Modern criticism?
Perhaps because of its relative inaccessibility (unless one has special access to the Vatican, the sculpture is only visible from one viewpoint, under the portico of St Peter's), the statue of Constantine has received little art historical attention. This also may be due to the quality of received opinions about the quality of the sculpture of itself. Lavin passes over the work in his Bernini and the Unity of the Visual Arts, and Avery (in his "Bernini: Genius of the Baroque") says very little. Even Marder, the only art historian to concentrate an entire book on Constantine and the Scala Regia, admits that "We may never know what the equestrian statue of Constantine looked like before it was finished in situ, but one guess is that it possessed greater formal power in Bernini's mind". Rudolf Wittkower is much more generous  - "The monument is placed in front of an enormous wind-swept drapery, and this device enhanced the beholder’s belief in the sudden interruption of the rider’s progress and make his fascinated reclining and the reactions registered simultaneously by the animal an extraordinarily convincing experience."
Scala Regia Rudolf Wittkower