They've ended the directors of Avengers 3 and 4.
They're now universally so hyped, you can't say **** against them. This  thread and past versions of it is proof. Their following is as devout as  Joss Whedon's and Chris Nolan's.
		
		
	 
And this happened because they have made good movies, not because they used only already popular characters (which they didn't).
Where is Whedon now with his love for the popular Black Widow?
And what do you want from fanboys? They can never stand any criticism of their beloved movies. It's not only the Russos' case.
	
	
		
		
			Sharon isn't an Avengers character, it isn't Whedon's responsibility to develop her.
		
		
	 
He had Hill and Dr. Cho at the party in the Tower. And he was the one, who made Falcon being an Avenger.
	
	
		
		
			Is it Feige? Why haven't any other movies played host to other Avengers?
		
		
	 
Because MCU is a constantly evolving thing? Look at Thor 3, Homecoming. 
Crossovers are gradually replacing solo-movies.
Also, Cap1 wasn't really a win at the box-office.
Marvel gets money from it. Not the Russos. That's the point. Their responsibility is to make a good movie, that's all.
Also, very important note: it was the Russos, who made Cap's character popular and interesting. Before them not so many people really cared about him. They deserve this credit. 
	
	
		
		
			They have no responsibility to do a lot of things like develop Black  Widow, or introduce Black Panther, or turn Scott into Giant Man. They do  it because they'd rather do that than do Crossbones, Zemo, or Sharon  justice. Or to give Sam scenes for his own sake. Or to have Bucky being  anything more than a source of strife and put in another fridge when  that's done.
Or, you know, to give Steve a character arc.
		
		
	 
I'll give you Black Widow, she is obviously one of their beloved characters, but Black Panther was introduced because of the need to promote him before his solo-movie. It's on Marvel. Ant-Man was there only for the airport fight, very small role, and they made Zemo interesting, gave him a decent time and many people said he was one of the best marvel villains. What's wrong with him? That he is not like in the comics? I'm sorry, but MCU has long gone far from the comic-book's canon. And he was left alive in the end, maybe he will get his iconic look later.
Crossbones. I would say this falls under the general marvel problem with their villains. Remember Strucker? With 1 minute of screentime and killed off-screen? Whedon played him much worse.
Also, iirc, Grillo said recently that he asked to kill his character himself, or something like that.
I think you are a little too biased against the Russos. I'll give you Sharon, they've made a big mistake with her character and this is really a problem. But still the Russos were the ones who made Cap, Sam and Bucky really popular. Sam got his scenes in TWS. Bucky this time around has got TWS!Black Widow-sized role. It's very obvious, that they are very fond of Sebastian Stan and his character, they constantly praise him, talk about Bucky being such an interesting and tragic character even more than Cap himself.
And gosh, Cap had his character arc in both their films. In TWS it was about letting go of his past, finding himself in the future, where he doesn't know who he can trust and staying true to his moral compass, no matter how dirty the world around him is now.
In CW there were many things too, but most importantly it's about him letting go his Captain America alter-ego and finding out who Steve Rogers is by his own.
I suppose it's not very obvious, because he is such a good guy, he can't fight with his demons like Iron Man, he doesn't have much of them anyway. But still we found out his flaws in CW, that he is human, not a saint angel. I think, it's a quite interesting development.
	
	
		
		
			Because she didn't really bond with any of the Avengers before, so why  would Tony hire her? In CATWS she helped Cap, so her move to help the  rest of the Avengers was more palpable.
		
		
	 
Why would Tony need her bonding with anyone? He hired her for a job, not to be anyone's friend. He already knew she is competent from the Avengers.
Really, Marvel usually don't bother to explain much more important things than such a little detail. They didn't even bother to explain properly how team Cap was found in the airport and everyone was cool with it.
	
	
		
		
			I think they did, but the Russos have kept whittling away at her to service other parties.
		
		
	 
The Russos ordered them to write her scenes, which were included in the movie, so poorly? Or did they rewrite them on purpose? Or did they cut all the best scenes for some reason? It's not only about time. Look at Ant-Man, Spidey. They have little time, but still what they have became fan-favorite bits. It's that her scenes are boring and uninteresting and make her look bad.
I've already described one example with the kiss scene. With the same amount of time this scene could have been written MUCH better.
Instead of showing her own personality the writers go out of their way to ensure that the audience will  mentally slot in Peggy's personality, her time spent building trust in TFA. Sharon offers Cap a from beyond the grave talk from Peggy. She doesn't have anything to say from her own. The time of her dialogs and monologs is spent talking with Steve about Peggy Carter,  specifically about how Peggy encouraged Sharon to enlist, bought her a  thigh holster, and how Sharon didn't want to force Peggy to keep secrets  from Steve. Instead of spending time building up the imagined  chemistry between the two they use Peggy Conversation. And after that they blame the audience for suddenly wondering if Steve's become confused as to who he thinks he's kissing  Sharon or her aunt?
In SteveSam dialogs in TWS we can see why they've became friends, what they have in common, the bond between them. About what exactly are the dialogs between Steve and Sharon when they are not talking about Peggy or how Bucky almost killed her? About lies. 
	
	
I mean, as I've said, 
something happened in pre-production. Initially she had a bigger role, she was in the airport. What happened? I don't know. Maybe VanCamp had got some illness and couldn't have filmed much? Maybe the reason why they cut her role in CW will affect her possible role in IW? Maybe it's not all up to them.