Well obviously you can't use a whole pork injection on a butt.
This is because the snout is still attached on a whole hog.
The enzymes from the snout melting into the meat cause for an "Off" flavor throughout the entire hog.
Whole hog injections counteract this souring of the meat, by adding an element called leaverite.
Leaverite is actually mined throughout the United States. Many gold prospectors around the country have been known to come across large quantities of leaverite.
If you were to use a whole hog injection in just say a butt the consequences could be quite disasterous.
Being as that there isn't a snout, the active particles in the whole hog injection would be racing around trying to find the snout souring element.
The action of these particles racing around in the meat, not being able to do their job would just create friction, thus heating the meat faster than normal resulting in higher cooking temperatures, and thus tougher meat.
Sorry, I need a day off!