As someone who has used a ton of self-generated Eevee codes, I don't think that warning is totally accurate. Using an unofficial code just stops you from being able to use another code that shares the same password flag (slot) as the one you generated. So if you used the official Eevee one (slot 1) then any code you generate for slot 1 won't work. Whereas if you generate your own code for slot 0, and that's the only code you've ever used, you can still use all 63 of the other slots, however the official codes aren't exactly all in order, so generating them (for personal saves) can take some trial and error if you have already used official (or unofficial) codes. Though my use was on an actual cartridge and emulators may act differently toward them. But that's basically just an explanation of how the password system works.