This thing is outdated. Just use GBA mode on old 3DS/Old 3DS by using open_agb_firm and if you want use mGBA in New Nintendo 3DS/2DS as games usually run mostly fine in the emulator in that specific hardware.
Also the 3DS/2DS has a limited number of "slots" for games so by using the injector you are occupying those slots for every GBA game, while by using open_agb_firm you use no slots and by using mGBA you use a single one.
So overall the only reason to use the Injector nowadays is if you plan to play a limited number of GBA games and want it to look nicer.