The problem with that logic is, Ronaldo has also struggled at International level or at the very least, hasn't hit the heights he has in club football. You could argue with conviction that he has failed to deliver on many occasions against the best sides in both club and international football, whereas Messi has NOTHING to prove at club level. He's broken Barca records, CL records, European records, the lot! International football is a different story, I agree he still has something to prove. But then again, if he ends up scoring 800 goals, wins 5 or 6 European Cups, 10 league titles, countless other cups and multiple Ballon D'ors and World Player of the year awards, then can there seriously be any debate about whether Messi is the best?
I don't think it's sensible to say that Ronaldo has carried a team that has the likes of Ozil, Alonso, Higuain, Di Maria, Benzema, Kaka and Callejon. They're not exactly West Ham! Real Madrid won the league 4 years ago ahead of a Barca team with most of the players that play for them now.
It's not fair to bash Messi with the haven't proved it in other leagues talk either, why should he leave Barca? No one questions Pele's achievements even though he only played for Santos and never played in Europe.