It only bricks it on lower firmwares, and it is not only SX, it's every cfw you would use to load a lower firmware.
The cartridge controller has its own set of fuses and firmware basically, so when it gets updated a fuse there is burnt, rendering it inoperant with lower firmwares that require a lower cart-fuse count to load the cartridge reader firmware.
There are some protections in some cfw to forbid this controller from updating (by deactivating it), but that means you then cannot use cartridges in newer versions of the OS that need the updated cart slot, and that is only of use for people that want to keep nand on a low version and update emunand to play newer (pirated?) eshop games there.