Marciano is VERY underrated as a champion, Ali had mad respect for him
he had the heart of 2 lions, and an amazing work rate combined with crushing power
Marciano could have competed in any era - even the 70's , he would have bulked up etc...
Styles make fights and certain guys would have given him trouble for sure - Foreman, Liston,Frazier, Tyson come to mind real quick, though he could have beaten all these guys -it's possible, maybe not probable, but possible - Foreman maybe the one with the biggest odds against him