Mayock said Johnson, essentially, is overrated because he has a tendency to have problems shedding blockers and such. He said as long as hes free to run and make plays relatively unimpeded hes great but he gets caught up too easily.
Also, I too have doubts about Benson. People look at his production in terms of overall yards but when you look at his APC each year its not so stellar. Texas, against most schools is a mismatch talent wise and yet he still doesnt have a very good APC for an upper-echelon RB...and it basically boils down to speed. He doesnt have enough long runs to stretch his APC, like you alluded to. In the pros a lack of speed becomse more pronounced. He has great durability and might do well on a team with a good Oline but Fason looks like a better prospect than Benson. Maybe Im leaning too heavily on the APC thing but I think its telling.