(Images stolen from my previous tutorial)
With a traditional MenuHax CFW coldboot setup, the system boots sysNAND before loading MenuHax and entering CFW, so if sysNAND is dead then MenuHax can't run and it's no longer possible to enter the Homebrew Launcher to load recovery tools:
But with A9LH, tools can be loaded before sysNAND even starts up, which means that it's possible to open Decrypt9 and restore a backup in case of broken sysNAND because sysNAND doesn't need to start first:
A9LH will rely on FIRM0/1, however CFWs like AuReiNAND protect FIRM0/1 from being overwritten, which means that so long as you have AuReiNAND set up and don't deliberately poke around these FIRMs, you'll be safe from bricking or losing A9LH, even after updating sysNAND.