^^^ +1 the hardest thing is the bolt patterns on the hub/sprocket/rotors. Spacing between forks and swingarm can be fixed with spacers, but if you have to use a whole new sprocket carrier or rotors, you then have to change calipers.
And on the front, that could mean different calipers, then forks, then triples, just to get the wheel to fit, not to mention you are designing the handling of the bike around the looks of a wheel, rather than by handling characteristics first.
2009 Aprilia Tuono - Ginger
2001 XR650R BRP (Big Red Pig)
2006 Honda 599 - Ex wrecked it :-D
2007 Honda CB900F (sold)
2006 Honda VTX 1300C (sold)