The main issue is the firmware being stored in the same flash memory the rest of the data is, so it's subject to the same corruption. "bit rot" in NAND flash is real - the stored charges will gradually leak out, and especially with the shrinking of each bit cell and MLC, even data that's sitting there dormant will gradually erase itself so the firmware has to periodically rewrite these blocks, at the same time moving them around to do wear leveling (another thing that has been sacrificed in the higher density shrinkage is write endurance.)
Corrupting data or BMTs if powered off in the middle of a write is not such a bad thing compared to if that data happens to be the drive's firmware. In the former case at worst you lose a superblock and the OS doesn't boot, but you can still recover from that fairly easily compared to the latter case, where the drive can become no longer responsive.
It's unlikely that the firmware region will be corrupted unless you are in the middle of a firmware upgrade. While with the variety of SSD and HDD failures I've seenis large and such a failure is not impossible it's one I've never seen and sounds rather improbable.
Corrupting data or BMTs if powered off in the middle of a write is not such a bad thing compared to if that data happens to be the drive's firmware. In the former case at worst you lose a superblock and the OS doesn't boot, but you can still recover from that fairly easily compared to the latter case, where the drive can become no longer responsive.