Undefeated records are deceiving. It can either mean a guy is that good, eg Marciano, or he didn´t fight the best at their best, eg Calzaghe. Depends on the case.
Depends, Marciano's undefeated record isn't that impressive, given the first time he stepped up was on his 26th fight against LeStarza, which he arguably lost, then stepped down his competition until his 36th fight when he fought Layne, then he'd only fight at the top level for 4 more years and retire. I mean it is impressive but not as much as many make it out to be. Arguably less impressive than Calzaghe and Mayweather
In all fairness, Calzaghes opponents apart from a few could hardly be called Championship lvl quality.
Marciano is leagues ahead of Calzaghe in terms of the quality of their record. Mayweather is more debatable.
It's not how long you fought, it's WHO you fought in that time period. Marciano clearly beat better fighters. This is not up for discussion.
If a guy has an unbeaten record it shows you that he has not lost a fight. In theory anyway. Might not end up like that with gift decisions and such.
Marciano was the undisputed champion, Calzaghe was WBO for 10 years undisputed for only 1 year, big, big difference.