The success of a TV show and the success of a movie are two different things. Firefly and Dollhouse didn't die because they were bad shows, they died because FOX doesn't really know what to do with shows like that; they don't know how to sell high-concept science fiction that isn't dressed up as a procedural. You wanna talk about success rate, look at the overall success rate of shows on FOX. They cancel everything.
Whedon knows comic books. He writes comic books. He has the proper sensibilities to approach a superhero franchise. Having him direct the Avengers movie is a great idea, because he already has an understanding of the characters and relationships. Avengers will be successful because Marvel knows how to sell their movies. Marvel Studios knows what they're doing; this entire Avengers movie franchise is brilliantly laid out, and is exactly the way a team-up movie like Avengers should be done: introduce heroes in solo adventures and then make a big-ass team movie.
Whedon will put out a quality product and Marvel will sell it.