I'm not sure how security is implemented in iOS and whether or not it is directly comparable to this scheme. UEFI secure boot only validates the bootloader, everything beyond that is going to be OS specific and out of scope of UEFI. Other systems have implemented this kind of cryptographically protected security, such as the Sony PS3, and have been very resilient in the face of persistant attack. AFAIK the PS3 has only had one successful attack in 5+ years, and that wasn't able to persist on the system, the secure update mechanism remained intact and was able to clean off the exploit and prevent re-exploitation. I would hope that our rockstar Linux devs could build a system at least as resistant to attack as that, if not more so.