Brute and C.Miller hit the nail on the head... a catch dog catches a hog... period. If they ever back up and bark, then they are just rough bay dogs that will probably catch 95% of whats out there. I hear stories all the time about people getting on bad hogs and their "catch dogs" back up and start baying... those aren't catch dogs!
My old Clifford dog was originally used as a RCD... until he started going out and finding his own hogs. Now he's strictly lead in.
Most RCD's that i've seen are crossed with cur dogs. The hardest catching dogs i've ever seen have been cat/ab or cat/dogo crosses.
I've seen pits that will bay the hair off a hog, AB's run back to the truck and dogo's bark once and piss on a bush... just because its a bulldog doesn't make it a catch dog.