O geez.  Where to start. lol.

Case inserts.  The OEM case inserts do not expand at the right rate causing the main bearings to fail and suck air in the motor.  I've seen a few fixes for this.  Brass inserts were one but i believe they weren't the best fix?  Seems like builders were using a different metal that worked better. 

Flywheel rivets fail.   Better get that hub welded.  Suzuki tried to fix it in the later year zillas by using a larger hub.  I believe they still fail.  I prefer using the smaller hub ones since they are much lighter.  As soon as you start adding HP to the zilla, the flywheel WILL fail if you skip welding it.  Seen it on a couple of my buddies zillas.

Ill more people chime in about the rest.