I agree that Ronaldo just doesn't get criticised. I think it's because of his goal record, his brand and what commercial value he brings to a league. For the last few years he's basically Fillipo inzaghi (or Jermaine Defoe/Darren Bent) just better in the air. A goal scorer and nothing else. He stands up front, does nothing for 89 minutes and gets you a goal at some point. You can have him in the side but it means the other 10 players have to work hard to compensate for him, or at least the two banks of 4 behind him. United don't have anyone in the midfield who can do that job, that's the problem. Pogba, Rashford, Greenwood, Penandes, Sancho are all attacking players and then fred and MCTominay, Matic are no where near the level required.
TL:DR; Ronaldo should have gone to city. He would have been perfect for them.