I don't think I agree with that. I think that his work stacks up well against other actors of his generation. He was a studio contract player and worked as a young actor with Rod Sterling (Twilight Zone), in screen adaptations of stage plays (The Ice Man Cometh), Network television guest roles like Alfred Hitchcock, Route 66, and Perry Mason to name a few until he was cast in big screen productions and was an "A List" actor for many years. He, IMHO, made some extremely poor choices of films when he had a chance to get studio execs to produce what he liked, but he is a fine actor by many people's standards.
I might put him in the same boat as Sean Penn, George Clooney, Rob Reiner, even Cher. Celebrities who's hubris and idea of self importance have allowed them to think that they can use their own public personae to tell others how to live.
He was just awful in “The Great Gatsby”, but his career kept growing. Even in “The Natural”, his work was just average. Funny, but now that you mention it, his best work might have been that episode of the “Twilight Zone”.
It's just not his politics. Lot's of very good to great actors from his generation, Jack Nicholson, Robert Deniro, Paul Newman, or even modern day, Sean Penn, I strongly disagree with politically. But, I still recognize the quality of their work. In Redford's case, I'm pretty sure that the guys looks dictated his career success. But, like I said, it's all a matter of opinion.