Ted Tso detailed the protection of the journal in this thread (I've deleted the particular message or I'd quote it for you)
I'm not sure I believe that parallel fscks on partitions on the same drive do you much good. the limiting factor for speed is the throughput of the drive. do you really gain much from having it bounce around interleaving the different fsck processes?
as for protecting against this sort of corruption, I don't think it really matters.
for flash, the device doesn't know about your partitions, so it will happily map blocks from different partitions to the same eraseblock, which will then get trashed on a power failure, so partitions don't do you any good.
for a raid array it may limit corruption, but that depends on how your partition boundaries end up matching the stripe boundaries.