M&P Slide milling for red dot, possible on all models?

[Sllarson55]

I contacted a cerakote applicator that also offers cnc slide milling for red dots. Seems to be a very reputable company. I have two M&P slides that I want milled and cerakoted. He mentioned that not all M&P slides can be milled because of differences in the area being milled and that he would need to inspect them. Is this common knowledge? I've never heard of such and couldn't find any info from an hour of searching online.