#1 Lesser competition
Despite facing some intense competition from Napoli last season, Serie A still remains a one-horse race, and now that Ronaldo has joined The Old Lady, the league and cup title almost looks done and dusted in Italy, it will be a major upset if Juventus fails to claim any one of those titles.
Despite the fact the Serie A was arguably the best League during the 2017-18 season, it is still not as competitive as La Liga. The teams from La Liga are far better than that of Serie A. Real Madrid in particular faced stiff competition from Barcelona and Atletico Madrid for the La Liga and Copa Del Ray titles.
During his 9 year spell at the Bernabeu, Ronaldo only managed to win 2 La Liga and 2 Copa Del ray titles. He also had to face Messi, and more recently Suarez and Neymar for individual honors.
Serie A has no such competition, and thus it will boost Ronaldo's chances of winning multiple club and individual honors.