Electroplating traps hydrogen ions within the nickel-to-brass interface and within the crystaline structure of both base and plating metal. It is called "hydrogen embrittlement", and for critical applications, preventive measures must be taken to remove this hydrogen. For example, for hard chromium plating onto steel as in aircraft landing gear forgings, the plated object is heated in a carefully temperature controlled oven for several hours to release the trapped hydrogen as gas. If this is not done, the entrapped hydrogen migrates, causing stress concentrators that enable corrosion to start. Not good for long term durability and a flight safety item. Similar treatments are used in other aerospace applications, as well as for objects exposed to marine corrosion.
With cartridge brass cases, part of the preparation for plating involves an acid "pickle" bath to prepare the brass for the plating. T bathhe pickle also thins the brass microscopically, and allows some of this hydrogen entrapment. As far as I know, nickled cases are not oven heated to relieve hydrogen embrittlement, because the product is not considered to be a durable, infinitely reusable item. Therefore, it tends to develop case mouth cracks more readily than non-nickel plated brass.