This is how i see it. How many NFL caliber edge rushers did these guys face?. The dropoff is huge in edge rush talent from certain conferences.
You'll see OL scouting reports on some players and it'll mention how he did against one SEC school his team played against. And even if he held his on in that game, how would he have held up week after week facing that level of competition?
Tough to judge. To me Campbell's biggest plus is his endurance and health through a long and productive college career. I think he'll be a good pro, but maybe as a guard.