A brass insert moves a bit more of the weight rearward inside the shaft, thus reducing the FOC a bit, probably .25%, not enough to worry about. Similarly, a longer broadheadof the same weight as a shorter one will move the FOC marginally forward.
If you're using carbon arrows, you generally have to use an aluminum or brass insert, into which you screw an adaptor mounted with a broad head. Brass may be a tad stronger, but not enough to worry about on deer size critters. Regardless, usually the adaptor bends before anything happens to the insert.