It doesn't matter if you're white, brown, blue, or purple as long as you have the right athletic parts. I agree that it's rare to see the right parts for a RB/CB in a white athlete, but it can and will happen. I think Gerhart is a good example. He's 6'0 231 with 4.50 speed, a 38" vertical, and a 6.94 three cone time. By NFL standards, he's not a GREAT athlete. But he's a pretty good one. His workout numbers absolutely blow away those by LeVeon Bell, Eddie Lacy, Carlos Hyde, and Jeremy Hill.
If you're going to say he can't be successful, you're going to have to find a better argument than, "His skin is white."