The overall result is good, but there are some finer details that aren't quite right... From what I remember seeing it was issues with the faux-whatever, be it aging, marble, rust, etc.. They just weren't executed well. The marbles on the columns in gringotts were laughable and the "old" signs and adverts arouns the area had issues with color matching. One technique is to paint it like it's new and then put the under-color on the top to make it looks like it has worn away.. which works with things like stencils and sign lettering.. if you properly match the color. The letters on the alley wall next to Magical Menagerie were a blatant example of this. The color that was supposed to be the background color of the wall was just sort of flung back on top of the letters, didn't match and looked like the paint splatters it actually was instead of portions of the lettering fading away.
Paint of the wrong color and of the wrong consistency and applied the wrong way.