Its a look we love. Exposed brick. Few Scenics can really master the look without creating a cartoon look.
The biggest problem I have stumbled on has not been the finish, or the technique. Often the scenic work is very well executed. The problem lies in the placement of the rot.
The plaster crumbles and rots due to moisture problems in the wall. This usually happens at the upper corners or the lower sections in the wall. Sometimes it occurs along a crack in the wall, where the plaster cracked when the building settled. It is rarely 'just in the midle of the wall'. There needs to be a story behind it.
Another tell tale sign of authentic broken-away plaster is the bricks that are eventually exposed, are often also partly rotten away. They were rotting behind the plaster for years before the plaster crumbled off. The mortar can even be proud, standing out like a protruding web.
There's great reference on line. Google Images: "brick+ crumbling plaster". Have a look. Its actually pretty easy to tell which images are scenicly done, when you know what to look for.