Just be sure you know how good your Magic PCs are and how good your NPCs are after the aspected background count bonus and negatives are applied. High background counts can cripple mages to the point of uselessness if not balanced carefully. Its part of why bugs are scary and all, but you always gotta try and let the PCs maintain at least minimal effectiveness unless its a specific plot point, or they just insist on doing something very very dumb.
In my games everyone once in a while I like to give the PCs a glimpse of something big and bad, but I generally take steps to make it very obvious this is a Big Bad and they are not prepared like having them watch the Big Bag eat a couple HTR team members in full HEAVY Millspec armor while taking multiple Assault cannon rounds to the chest at point blank range while not even being staggered by them. I dont do it alot, but it can be fun every once in awhile to remind the PCs there is always a bigger fish.