While mechanics and footwork are important to a quarterback's completion percentage it can not be the only thing that contributes to that statistic. Andrew Luck has been called the most fundamentally sound quarterback and the most NFL ready quarterback in terms of mechanics to come out college in a long time. However, for all his great mechanics and talents as a pocket passer, Andrew Luck so far has a career completion percentage that is lower than Newton's at 58.6 percent. When Luck broke Newton's rookie passing record (4,051 yds), it took him 110 more attempts to set the new mark at (4,761 yds) the following year. Cam Newton has thrown 54 interceptions over 4 years but it appears that Luck is on track to throw more interceptions in the same time span as he has thrown 43 interceptions in just 3 years in the NFL.
Mechanics are very important but either Andrew Luck's mechanics are over-hyped or concern for Cam Newton's alleged poor mechanics are being overblown. Cam Newton ranks
7th all time in quarterback rating (85.4) for rookies that came in and started their first four years. Luck's ranking will not be known until he completes his fourth year but is not currently projected to surpass Newton's first four year rating unless he goes out and has his best year ever thus far.