When you ride the cap, you have smaller angle that is more efficient for cutting, does more skin scraping if you apply pressure, and takes a toll on blades harder than steep angles (perpendicular to the face). I'm guessing for someone with really coarse beard a Feather will cut well even at steep angles where Derby will pull hair. But if you adjust to a smaller angle, the Derby will cut well just as any blade.
I find razors, blade gaps, angles and any geometry in shaving more important than blade sharpness. They all pretty much cut the same to me. I've shaved with cheapest Chinese blades that I got for free in $5 razors and they worked brilliantly. My beard is medium coarse, enough that I must use beard oil after shaving or else the goatee becomes a Brillo pad. I toss blades after 3-4 uses, 4 being noticeably problematic.