While a low job level onion knight won't do as much damage, I thought a 99/99 FEOK would do more damage than a 99/99 Black belt?

Monks are ok early on, they have moderate HP growth, not so great armor, and are just a few hairs weaker than black belt in the attack category. I think a BB has 40 or 60 more atk than a monk at j99, but I can't remember.. as I don't have a Monk at 99.

Monks have retaliate which is 2x normal damage counterattack... that can be good if you have lots of attackers attacking your party.

High barehanded attack growth, slightly higher hp growth than classes at his same time of acquisition, low armor, and a 2x counterattack.