There are two concerns. First, if power drops during a physical write operation, that sector is scragged. If it was writing metadata, you have serious problems with whatever files that metadata describes, if anything points to that sector.
Second, more subtle but probably more important, drives lie about what is physically on disk. To look good on benchmarks, they tell the controller that sectors have been physically copied to the platter while they are still only in buffer RAM in the drive -- up to several megabytes' worth. A few seconds after the last controller operation, these writes have drained to the disk. Before that, there's no guessing which have been written and which haven't, and blocks the system meant to write first may be written last. As a consequence, after powerup the file system sees blocks that are supposed to have important metadata in them with, instead, whatever was left there.