Kinda a mix of everything. It's worth noting that, according to ex-kernel hacker Matthew Garrett, you can achieve the same bricking using a 20 line program in Windows. So it's not a Linux (or systemd! Seriously, don't we have enough hate against systemd without TFS adding fuel to the fire?) issue, it's more a design fault.
Clearly UEFI variables are expected to be written to by suitably privileged programs under consumer operating systems, otherwise Windows and Linux wouldn't expose them the way they're exposed. Yet clearly variables are being exposed like this that shouldn't be written to under normal circumstances.
Why is it so big and ugly?
That's what she said