I think its fair to say that Bradley had a hard time with Felton...but I wouldn't go so far as to make a blanket statement about who he can't guard in the NBA. Give me the facts and I'll buy in, but in the games I've watched he's been a pain is #$$ for 99% of the players he's matched up against. (D.Wade is a big 2-guard who would probably agree with me).
I think the Felton issue was an abnormality. Bradley wasn't putting the defensive effort we were used to seeing, and it was quite obvious to me. Part of the reason was that he was focusing too much on doing things well running the point in Rondo's absence that he simply wasn't focused defensively as he usually is... in part, he wasn't pressuring the ball, which is his main weapon.
If Bradley actually defended well, and Felton got his, then I'd give up the point. But this was more of an issue on how Bradley was playing than anything else.