Plot hax or not, it happened. Don't use it as an excuse for not happening.
That being said, only J-Man would have stood a chance against that thing due to sage mode. Itachi's mode was especially adapted so that those not physically adapt would be defeated within seconds. We haven't seen a battle with Sassoco where the other ninja didn't have a overwheming ability to compensate for it's sheer power. Hence it's fair to say that J-Man is merely adapted to dealing with such an ability, but not adapted to being able to defeat it.
Personally, they fall under entirely different roles. Trunaide was always the weakest out of all of them when it came to simple combat, but she was unrivalled in her healing abilities that enabled her to heal an entire village and the Kages on two seperate occations, it is for certain that both battles would have gone much less favourably if a ninja of her expertise didn't exist. She also adopted the medic for every team, which in theory should and does keep ninja living for longer.
Oro created a entire village and pushed the boundarys of science, learning many many jutsu, advancing the Edo and uncovering the contents of the Stone Tablet without even being able to read it. He was a genius on so many levels and had the motives to use it. It was that that enabled him to use the entire Sand village to attack the Leaf, and have labs dotted all over the land despite being a listed missing nin. The only thing I don't really get a feel for is how capable he personally is, but his edo was used by Kabuto to amazing effect, something he personally set the groundwork for.
J-Man doesn't have accomplishments on those sorts of scale and that is something that he is aware of in life. He is a legendary wandering ninja with the greatest physical ability and is known for getting in places where he's not wanted (heheh), but on the same token he's merely specialised in what he does and what he teaches.