I'm not worried about his 1 on 1 defence tbh. Couldn't be worse then nius. But that costs 1 try a game, and not certain to save them if you are great as a fullback 1 on 1.
The problem is his communication, positioning, anticipation of play and counting numbers from fullback. You don't get better at that playing on the wing, no matter what level you play at...
Those easy run throughs from 2nd row by the panthers weren't just poor defensive efforts, the numbers and line position were all wrong and they happened once cobbo went back there.