Well not really, you just need to know what FIRM was installed at the time of the brick, and even if you don't, you could just systematically go through each firm until you find the right oneI think it is possible to do that now, but you need to have the xorpads for that 3ds' nand. Mostly the firm0 and firm1 xorpads.
The only 2 bricks sighax would not be able to fix is, a 3ds with firm0/1 both corrupted with a unknown firm that was not stock (like if the system crashed mid FIRM write) or a system with no original nand backup that has had the FIRM partitions overwrote I.e if someone used decrypt9 and restored a nand backup from a different console and made no backup prior to doing that
.....well and all the other types of hardware brick, I.e broken components or mcu bricks
But yeah my statement was in regards to people who already have a9lh, people who are bricked, or people on 11.4 are a different matter