Affleck will be OK, it is what they come up with as a script that will decide if the film is any good. Making this work right on the big screen is incredibly hard, and it is not like they can get Joss Whedon.
If Nolan is in charge I'd expect some good leadership, but they also need a great script and a good director. Being out alongside and available for direct comparison to the Avengers 2 will not help them.
That said, the absolute worst that can happen is a shitty movie, so they can knock themselves out.