The regen and him being a master CoC user was the problem. He would beat any top tier handily.
Him being a CoC user without the regen isn't an issue, since Gaban and Shanks were slicing and dicing him + fodder giants. But with that combo as you highlighted, yea of that time you would need two top tiers to put him down, which only the Marines had. But since he's working with the Marines, Roger Pirates no longer exist, Whitebeard had bunch of children as crewmates, nobody had the fire power to stop Harald, Gaban's assessment is correct in that sense, if Harald being backed by the Marines & WG is taken into account.
But yea the combination of it is an issue that, I don't think anybody would be able to counter it 1on1, except... well i'll leave that agenda for another day, when the present day top dogs actually start going off.