A bit of both really. In his 45 fight career he went 40 wins with 38 by KO and 5 losses, all by KO. Also, he only saw the final bell in 3 of those fights, so his power saved him in 38 of them and his chin let him down in 5.
Hard to pick with the above information. Just how many of those wins may have gone the other way if his opponents took him deep in fight to the final bell------we'll never know.
I don't think his chin was as terrible as people make out. It's more a lack of boxing fundamentals, technique and IQ. You're feeling it if a top heavyweight hits you flush. Bruno was very raw and powerful. Muscles and might over boxing ability.
He gassed so bad during the McCall fight. I picked stamina.