I would suggest that todays fighters have all the advantages. A greater understanding of effective training techniques, knowledge regarding diet and ability to analyse opponents through video and so on. If you pit a modern fighter against an older fighter based on these things, then maybe the olden times fighter might get shown up.

But I don't like to compare and contrast through generations because it is a different game today. Fighters of yesterday could go 15 hard rounds and more and were allowed to absorb a lot more punishment. Also top fighters fought more often. If Mayweather were to fight too often with his hand issues he might have a few more losses.