Okay, with Renekton it's something like 50/50, or 60/40 in favor of Renekton, but that's still very good compared to a lot of top laners. And of course he doesn't beat champions like Shyvana, she's FotM right now for a reason.
It's just that people seem to vastly underestimate Garen. He's a pretty tanky champion with quite the respectable damage. He's not as bad as people make him out to be. But that doesn't mean you blindly pick him into anything.
Champions who work against every other champion are usually pretty imbalanced anyway.