I know that the Avengers sold good but honesty the movie has nearly no plot at all. The movie just lived through the action it had and Ultron was way too much human.
It isn't so that you can't lunch a comic based movie when you have a good script and all the other stuff you need but when you have 5-7 X-Men movie before and 3x FF movie the audience has high expectation when you want to do a movie which sales good.