Ronaldo and Messi would score (more or less) as many goals as they are scoring now if they played in Italy with Real/Barca. They are the two best players who happen to be playing for the best two teams. Barca thrashed Napoli with five goals.
Defenders and midfielders will be much more brutal in Italy than in Spain. Teams will always look for ways to adjust, to neutralize Barca and Real as much as possible. In Spain nobody does that ffs.
In Spain scoring 7 or 8 against one team is allowed by the opposition. In Italy it's almost against tradition and the smaller teams won't be too kind in their tackles if they see a team pushing to completely humiliate them and score 7,8, 10 goals against them.
Barca and Real will almost surely have a similar number of pts at the end of the season, but the number of goals they score will be much lower thus Ronaldo and Messi will both score at least 10 goals less than they do in Spain. Especially Ronaldo will suffer in this league.
