What's "wrong" with it is that the post-processing makes the original animated frame look worse by obscuring or diminishing the positive aspects of the artwork. In the original base art, the colors are strong, there's a good amount of animated detailed in the water and there is an attractive quality to the way it looks. After all the post-processing, the water looks like generic 3D rendered "real water", the lighting crushes all the nice contrast in the coloring of the character, and we're left with a frame that has an extremely 2D looking character trapped between layers of realistic CG effects and backgrounds both in the front and the back. It looks nasty as a composited frame.
There's nothing officially wrong with that though. Obviously Ufotable was proud of it at the time, but it looks nasty. Art is about critique, and ymmv.