He was good at real madrid but it's not when he was on the MU. You must also see the the atmosphere of the club has been giving a huge impact to the performance. Majority of players in real madrid at that time when ronaldo was there have very big capacity unlike when he was on MU.
You might see how bad meguire and he is still become the main starter for the club. So, you can't compare the atmosphere of MU with real madrid.
Yes, his goals and success in Real Madrid was also due to the fact that the club already had some great players who all helped made the team a great team. Their abilities and contributions accelerated Ronaldo’s goal scoring era in the team. No can can deny that he’s a great striker and finisher, but he can’t do it alone. And that’s why he needs the backup of the team.
Manchester United didn’t have the experience in football players at Real Madrid had.