ACM is not the best material for flat cutout graphics. It tends to delaminate, though not always. Most vendors do not offer long warranties because of this. Nudo Products, for example, gives only a "5 Year (Limited)" warranty. They want you to put a trim cap on the material.
I would do ACM cutouts for someone, but make it clear to them there is absolutely no guarantee that it won't delaminate.
I have seen both ACM cutout letters and ACM routered faces delaminate, so I don't trust it. Also, not all ACMs are equal. Some of the first ACM makers offered a 15-year warranty on their panels, but 5 years is far more typical these days. There are "interior" and "exterior" grades. There is surely variation in materials among the makers—the skin thickness, the aluminum alloy, the finish paint, the adhesive, the core composition. Yet they all look frighteningly the same, don't they?
I prefer acrylic or aluminum for FCO letters, and .080" aluminum for routered faces. These materials have proven track records. Indoors, PVC or Gatorfoam works great. Of course, if you make ACM cutout letters that regularly last 10 years, then I would be comfortable using them, too, using the exact same brand and production method. But I prefer that others do the experimenting. I would not make a good test pilot.
Would the cut direction make a difference on your used material? Would a down-cutting bit be better than an up-cutting bit? Or would it even make a difference? I don't know.
Brad in Kansas City