Strum bar
The strum bar has been a problem for some players since the Guitar Hero series began, and the Wii version is no exception. Some claim that the strum bar only works when strummed down or vice versa, making it difficult for notes to be registered.
Detachable neck
The Guitar Hero III guitar was the first to use a detachable neck. This, in some cases, has made note recognition difficult as well. Some of the fret buttons might not work, while others do.
Fret buttons
The fret buttons themselves have also been an issue. They might get stuck and be difficult to press down. Some also have claimed that they received a static shock from pressing down on a particular button.
Star power
Since the series began, the Guitar Hero guitar would give off "star power" at a particular part of a song when the guitar is lifted at an angle. The star power on any of the guitars would either initially not work, or after countless playing sessions stop working.
Whammy bar
The whammy bar on all systems has been a constant issue. After some wear and tear from playing, the whammy bar tends to come loose and becomes unusable.