The difference with D3 is the entirety of inferno from act 2 onwards was like doing a nightmare dungeon 20 levels above what you can handle. You’d finally get to the point where you could farm act 1 inferno like a god, you’d enter act 2 and get one-shot by the first wasp fly as soon as you left town, and they would have so much health it’d take ages to kill anything, the difficulty jump was gigantic.
Made worse by the itemisation which locked ilvl62 items to act2+ and ilvl63 items to act 3+. In order to do act 2 you needed act 2 gear, and in order to do act 3 you needed mostly act 3 gear. Legendary weapons were all bad from act 2 onwards because legendaries were capped at ilvl61.
So the strategy was either to make a glass cannon build and try to avoid getting hit by any damage, or making a super tank and having “Just enough” damage to kill elites before they enraged, turning elite pack fights into epic long boss fights. Boss fights were like world of warcraft length raid bosses where you needed to optimise damage, self sustain and avoid mechanics in order to even stand a chance, and getting past Belial or Azmodan was a real achievement.
D3 difficulty at launch was nuts, because that was the endgame, there was no easy difficulty like in D4 where level 58s are running around killing lvl70 monsters in WT4 without much trouble.
Edit : I will give an exception to D4 though, Uber Lilith is harder than any of the D3 Inferno bosses, but she’s optional.