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.