1. World class teams.

Football has always been a team sport. No player is bigger than the team and even though an argument can be made in favor of these two superstars since their performances have led their respective clubs to unprecedented heights, the phrase will always hold true whosoever the player is.
As a famous quote says:
" It not the team with the best players that win. it's the players with the best team that wins. "
Messi and Ronaldo have always been blessed with an opportunity to play with some of the best footballers of their generation. From Xavi, Iniesta, Carles Puyol to Paul Scholes, Sergio Ramos, Toni Kroos, they were never short of quality around themselves, at least at the club level.
Although their stats are exceptional with their respective national team as well, with both being top scorers for their national sides, they have minimal success internationally. Although Messi has led Argentina to 4 international finals he hasn't been able to lift any silverware with his national side. And it was a fluke for Ronaldo and company to win Euro 2016, given the team was heavily reliant on Ronaldo to deliver the goods.
It's kind of like a symbiotic relationship between a footballer and his team, where both sides work together to achieve success. So even if Ronaldo and Messi have achieved so much personally as well as with their teams, it has all been due to the sheer brilliance of their team, not only their own performances, which has enabled them to make a mark for themselves in the footballing world.