Actually Cristiano Ronaldo can be a lethal weapon if he is in a team and playmates who have the same vision. He has proven that at Real Madrid, if you say his ambition can destroy his career I can't justify that, because in fact players play with big ambitions. I still hope he will stay at Manchester United.
I think Ronaldo would be doing decent in the short term in United, and that is why I would be shocked if he decided to leave the team and go somewhere terrible. At the end of the day, he is a good player but he could not really change a whole team just by himself anymore. His times in Real Madrid was until he was like 30-35 range and wasn't something big, then he left to Juventus when he got a bit older.
I am not saying that he is bad at all, he is good, he is even great but this doesn't mean that we shouldn't really expect him to score a bunch of goals and get the team to a cup or something like that, he would be just decent and we need to respect that by now.
Ronaldo changing club to another is not to help the club he wants to move to win Champions League but he is changing a club for his own personal interest. If any club thinks signing Ronaldo will help to win Champions League, I think that' is a wrong plan to make.