Not sure where you're getting that idea but when I google "Age of Ultron" a lot of stories about Whedon's bitching and moaning come up. I think he is causing the most rancor by whining about Disney making him do this or that. And I'm sorry Joss, but having Captain Marvel and Spider-Man appear out of the blue when Cap addresses the new team is BEYOND stupid. You should have kept your mouth shut on that because Disney did you a big favor by vetoing that. Now instead of Joss being the director of a somewhat flawed thrill ride he comes across as a big baby that the studio WISELY kept in check. I'm SO glad that he'll be gone from here on out.
But like so many things on the internet Whedon's bitching gives the aroma of blood in the water and the sharks are being drawn in.
I actually think all the complaining he's been doing has been making him look bad because the scenes he was fighting to keep in, most people thought were the weakest parts of the film, and the scenes Marvel were trying to put in would have fleshed things out a bit better. This is one of the few times where the director got in the way of the studio.
Here's another interview where he seems to be complaining about working with Marvel
Joss Whedon On Crafting Captain America's "AVENGERS ---" Line
"I made sure that we never shot Chris Evans saying [the 'assemble!' of 'Avengers assemble!'] I was positive that some executive was gonna go, 'You forgot to put in the last word!' I was like, 'With my dying breath...' I don’t have to say that a lot, but sometimes I’ll turn to [Marvel head honcho] Kevin [Feige] and say, 'With my dying breath...'
As much as I was like, 'We didn’t get this, we didn’t get this, this is sloppy, and I’m not happy with that music cue...' With all my complaints, it was in the script exactly as you see it. 'He draws breath to say the next word. Blackout.' So to know that we landed exactly where I wanted to go, however many stumbles along the way, was extremely gratifying. As was Chris Evans’ reaction when I told him what we were going to do in the second movie, at the MTV movie awards, when we won for the first movie – he lost his s***. (Laughs) It was great."
I read this interview and still don't know why it meant so much to him to cut out the last line. Really didn't make sense to me. I would have preferred if we heard Cap finished the line.