How much is too much, when it comes to using ferrites. Some of the boards I designed previously did not even have single ferrite. We pretty much never used ferrites. Compliance was not a problem. Now I design ARM9 based embedded system boards. There are ferrites for everything - NAND, DDR, CPU, CPU core. Are so many ferrites really needed? worst of all, there is no justification. They continue to use it just because its been used previously or it was used in the reference schematic. One ferrite after the power supply is great. But for every rail? It basically creates split planes. So no single reference plane for high speed signals. Makes layout hard, specially when DDR2, NAND are present.