Flash Memory limitation (Copy from "Embedded Linux Primer: A Practical, Real-World" by Christopher Hallinan)

To modify data stored in a Flash memory array, the block in which the modified data resides must be completely erased. Even if only 1 byte in a block needs to be changed, the entire block must be erased and rewritten. Flash block sizes are relatively large(For example, a 4MB Flash chip might contain 64 erase blocks of 64KB each.) compared to traditional hard-drive sector sizes. In comparison, a typical high-performance hard drive has writable sectors of 512 or 1024 bytes. The ramifications of this might be obvious: Write times for updating data in Flash memory can be many times that of a hard drive, due in part to the relatively large quantity of data that must be written back to the Flash for each update. These write cycles can take several seconds, in the worst case.

Another limitation of Flash memory that must be considered is Flash memory cell write lifetime. A Flash memory cell has a limited number of write cycles before failure. Although the number of cycles is fairly large (100K cycles typical per block), it is easy to imagine a poorly designed Flash storage algorithm (or evena bug) that can quickly destroy Flash devices. It goes without saying that you should avoid configuring your system loggers to output to a Flash-based device.