I'm finding a very unusual lack of info about those backup bootloaders, considering that flashing over the primary bootloader partition is one of the most usual reasons for people to brick, and the secondary bootloader can be used in some of those devices to just fix the phone in seconds..
On N4, the secondary bootloader only works with Qualcomm fast-download mode, but in the N5 the secondary bootloader is a fully-functional bootloader for fastboot that stays on the original bootloader version that came from the phone (in my case h10x), which literally makes you able to just zero the aboot sbl partitions and boot into the secondary bootloader and just flash a new bootloader and fix the stuff in matter of seconds.
I am not sure if on the newer Nexus there are also backup bootloaders (on N9 there are, don't know if fully functional for fastboot or no), but if there are and they're fastboot-functional, they're a HUGE deal.
Why there's like no info about this anywhere on XDA? Even trying to make a search here or over google for backup bootloaders brings veery small info about this, just isolated cases
I don't know if this belongs to a more technical forum or not