It can't be trained but boxers will be far more accustomed to your average boxers punch whereas MMA fighters for the most part aren't used to it.
It won't shock me if Mayweather stops McGregor early doors.
An awful lot of them honestly and sincerely do believe it.
A punchers chance is all he's got, his punchers chance will be considerably smaller than most of Money' opponents due to his lack of boxing craft.
I'd assume that your average boxer will have a more powerful punch and better punch resistance than a MMA fighter, one concentrates on punches most of the time, the other has to split his time between kicks,punches and grappling.