Looks like SysRq-o does a clean poweroff, not a dirty/immediate one — it calls kernel_power_off(), not machine_power_off().

This means, importantly, that can actually take some time to happen, as drivers get a chance to run deinitialization code. This also means that a wedged driver that doesn't respond during deinit can prevent the kernel from halting.

Thus, while SysRq-o might be useful for killing a wedged userland, it's not a panacea — especially, it isn't guaranteed to complete a shutdown for unstable kernels, or kernels with badly-written DKMS drivers attached. It's not truly equivalent to a power-cut.