I've had some pretty nasty brickings of devices, like overwriting the bootloader, that I've been able to recover from by getting it into some barely documented system on chip mode with a special cable, booting a new bootloader into RAM via the cable, and reflashing that way. One could go to the extreme and say any flash storage chip where all software bits are directly writable by a factory tool is technically unbrickable. But the customers won't see it that way.