Uggh. I tire of hearing that assertion, especially as vague and unsupported as it typically is. There's nothing in the SR setting that d20 can't handle. There's nothing about playing a shaman or street samurai or hacker tackling monsters and corporations in post-modern America that can't be done in d20. In fact, they already did it in
Urban Arcana to a large extent. And we just found out that d20 Cyberspace will be out later this year. Guess nobody's told the folks at WotC that it can't be done in d20.
No, you probably can't have a mage who's better with a pistol or sword than a merc, which is what everyone brings up right away in discussing a d20 SR conversion, but that's the system not the setting.