Modern mobile SoC's can probably be hard bricked even by accident. Run a kernel image that is not intended for the exact model of your hardware, and you incur the risk of e.g. misconfiguring some voltage regulators and trashing some arbitrary part of the SoC beyond recovery. (And the whole thing a huge mess because the patched kernel code is often the only kind of hardware documentation that's actually available for the SoC - hardware designers work directly on the kernel support, and implement all sorts of horrible, undocumented hacks in the process. Though I can only assume that the recently-mainlined SoC's and the IoT-ish designs that are being repurposed by the likes of Purism are somewhat better from that POV.)