It will engrave deeper at the high spots but not significantly so, not for me anyway :-)
Im not sure of the difference in hight from high to low spots on my plates Im engraving but its hardly anything, probably a nats chuff :-)
Mostly I get them dead flat but this seems to have solved it if they are off a smidg as I had no missed bits on this batch of buckles.

Im not sure how a commercially available one would work any different?