The "super capacitors" (almost nobody uses actual super capacitors after early models discovered that super capacitor lifetime at server temperature was inadequate) are just a low-pass filter - they usually only keep the drive online for a couple dozen milliseconds after main power goes down.
Most reasonable SSDs do not write cache at all, but thanks to the wear-leveling issues, they need to have a sector-mapping table to keep track of where each sector actually lives. That table takes many, many updates, and since it's usually stored in some form of a tree, it's expensive to save to media which does not support directly overwriting data (IE NAND, which requires a relatively long erase operation to become writable.) This table is typically what is lost during power events, and it is not usually written out when you sync a write.
So what happens is you write, sync, get an ack, lose power, reboot, and magically that sync'd data is either corrupt, or, even worse, it's regained the value it had before your last write, with no indication that there is a problem. This can cause some extremely interesting bugs.
Most reasonable SSDs do not write cache at all, but thanks to the wear-leveling issues, they need to have a sector-mapping table to keep track of where each sector actually lives. That table takes many, many updates, and since it's usually stored in some form of a tree, it's expensive to save to media which does not support directly overwriting data (IE NAND, which requires a relatively long erase operation to become writable.) This table is typically what is lost during power events, and it is not usually written out when you sync a write.
So what happens is you write, sync, get an ack, lose power, reboot, and magically that sync'd data is either corrupt, or, even worse, it's regained the value it had before your last write, with no indication that there is a problem. This can cause some extremely interesting bugs.