As far as I know a mirror is rather complex.
It has a glass plate with a “painted” reflective surface on the back, and the backside of that is usually painted black or brown.
This can be modeled it two parts; the glass, and the back plate with respective UV map for black and reflective sides.
But how would a Cell fracture treat such object as a complete piece?
When a broken mirror falls on the floor, you would see both front and back parts…