Every NAND is console-unique. It is encrypted using a per-console AES-128 key (the "NAND key") derived from the console’s private ECC keys. Without this key, a downloaded NAND file from another Wii is unusable without significant cracking.
Nintendo’s servers can detect mismatched NAND signatures. Using a downloaded NAND online results in an immediate permanent console ban. nand wii download
Some popular downloads for the Wii include: Every NAND is console-unique