Looking at Cornelius Johnson before the draft, he seems to have all the pure physical skills that you look for, but the issue is that he never really developed more of than the pure raw athletic skills. More of a great raw athlete than a detailed wide receiver.
That being said, I thought Johnson might be able to compete for the blocking WR role as he's physical and strong too, with 4.44 speed on top of that. But he needs a ton of development as a route runner.
Of course, Johnson got drafted in the 7th by the Chargers, now with his college head coach. Which just got rid of their two star veteran WRs, so they needed WRs and they didn't keep Johnson around on the PS. So that's a bad sign, but maybe he'll get better with development.