In fact, I think I'm one of the few people here that watches every single game in the season of Real Madrid, and most of Barcelona games. You can't say Ronaldo's only role is to score goals when he is the player with most assists ever in the UCL. Since he got to Madrid in 2009 he has 28 assists in the competition and Messi 20. He also helps going back a lot to help defend, and with the pressure to the defenders of the other team. Messi never defends, and a fact you can check on is that in almost all the Barcelona games, the player that covers less distance in the game is always Messi (and obviously the goalkeeper), that is a real fact bro.
Yes, Messi gets a lot of times man marked, because they can. They don't make the same with Ronaldo because it is impossible, he is sometimes left, sometimes right, sometimes as a striker, all in the same game.
What surprises me is that a lot of people here tend to forget how Ronaldo played almost all of his carrer in United and Real Madrid till 1/2 years ago and say that he is o ly a striker that is waiting a pass to score goals. He usually is involved in every goal of the team, neither he scores it, assists it, or launches the attack, you should know it if you see his games.
But to say that Messi performs worse in the UCL because he is man marked, with all the respect it looks to me like a cheap excuse for a player that some people want to name as the GOAT of the game. If you are not performing enough in the important games, change your role in the field and make your team advance. That is what Ronaldo made since he is getting older, he can't dribble or run as fast as he used to, so know he dribbles less and got better in his off the ball movement, and plays much more near the box. And it has worked marvels for him, 3 UCL in 4 years an 4 Ballons D'Or in 5 years. All because he keeps performing almost always in the important games in knockout phases. Messi disappeared vs Juventus last season, they went out. He disappeared in both games vs PSG (amazing when your team scores 7 goals in a game and you are involved in only one, and it was a wrong called penalty). A year before he disappeared vs Atletico. 2 years before disappeared vs Bayern, they lost 7-0 between both legs.
In all that games Barca went out, making them advance from the quarterfinals only once in the last 4 seasons.