Inside the SSD are flash memory chips. It takes a relatively long time (typically 10mSec) to tunnel the charge that changes a '1' bit (the erased state) to a '0' bit. If the power goes away during that programming time, the result in indeterminate. Not just bad, but bad in a potentially unknown way.
The worst case, which I've experienced with direct writing to flash chips, is that a totally unexpected flash location is corrupted. My guess was that the CPU started the write cycle just as power was lost, the CPU glitched the address lines as it was losing its brains, and the flash corrupted a random location.
Very, very bad.
If a power loss causes the flash to scribble on the wrong SSD location (e.g. the tables that keep track of good and bad blocks), the SSD "dies".
The worst case, which I've experienced with direct writing to flash chips, is that a totally unexpected flash location is corrupted. My guess was that the CPU started the write cycle just as power was lost, the CPU glitched the address lines as it was losing its brains, and the flash corrupted a random location.
Very, very bad.
If a power loss causes the flash to scribble on the wrong SSD location (e.g. the tables that keep track of good and bad blocks), the SSD "dies".