Agreed here. Comparing Messi with Ronaldo is not fair, as both of them are legends on their own, with their own set of specialties. But one thing that I would like to point out is that Ronaldo has always been like a one-man army, irrespective of whom he is representing. It happened when he played for the Portuguese national team, and the same happened when he plays for Al Nassr now. But overall, I have to agree with what you said. It will be wrong to compare two players with entirely different style of play.
It is not wrong to make such a comparison between two players who play in different positions when they play on the field, but for me it is just not accurate because apart from having different styles, they have never been on the same team. In addition, Ronaldo has often been more successful in the League he plays in, including being very successful in the Champions League and being able to win an individual trophy as the top scorer. While Messi began to shine less when he left the Spanish La Liga, so the comparison is not worth discussing if it is only about the two of them.