MCU Villains: Why so controversial when criticised?

Yes, they are. No one in real life is complaining about Marvel's villains. Just the internet.

People are too bitchy. Ultron was a great villain. I hope we see more of his type. He lived up to all the build-up.

There's nothing wrong with Iron Monger.

There's nothing wrong with having the hero face a dark counterpart of himself. In fact, it happens all the time in the comics.
Er, The Red Skull is Captain America's iconic villain. He kind of HAD to be the villain of the first Cap film. And The Winter Soldier/Bucky is another major part of Cap's storylines. He also kind of had to show up. And wait, I thought you liked Bucky.
Yeah, it kind of went without saying that Doctor Strange would be facing other magic users.

There's nothing wrong with evil Black Panthers. People are seem excited.