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Software Engineer “Toad King” has discovered that the Wii U GameCube controller adapter may be compatible with other devices, is working on PC drivers now.(r/Games)
I don’t know much of the Wii U adapter since the code has not been in wide use yet, however, I can give other suggestions.
The Mayflash adapter is the most commonly used and most commonly recommended GC-to-PC adapter.
However it has only two controller ports and is said to have slightly noticeable amounts of input lag when attempting fast techniques.
The other two options are the Raphnet GC-to-PC adapter or the Raphnet GC-N64-to-PC adapter.
These are much more expensive than the Mayflash adapters but are said to have less input lag. However, some people have said that one can configure the Mayflash to have the same speed as the Raphnet. No personal confirmation.