He was with Epic when it was making isometric shareware games. He was a big part of the reason Epic became one of the most influential AAA development houses on the planet, and that grew out of a startup. So he has already been there, he knows the path, and he has helped create some of the best and most influential games in the history of gaming.
I'd say he has a damned good chance at making it again considering his premier game with the startup is a multi-player shooter. UT 2004 is still a hell of a lot more fun than most of the multiplayer shooters out there these days. The reason UT III failed miserably is because the company released it broken, didn't patch it up to snuff quickly, and generally failed to support the game because they believed the PC was a much worse market than console.
Cliff is coming back to the PC, with a free-to-play shooter, which if he does right, will be a massive success on the scale of games like League of Legends. The reason PC was believed to be inferior by lots of developers was that people pirate games. But they can't pirate your game if it's free right? The game will be supported, and likely regularly release new content because he's not dumb enough to let it die. This might be the shooter people have been waiting for for 10 years. Not some shitty Console port, but an honest to god PC monster. I can't wait.