Harden's performance for OKC against the Heat in the 2012 Finals gets misremembered as him having some sort of all time bad performance.
With OKC leading 1-0 in the series, in a crucial game 2 Harden was 7-11 from the field in 34 minutes and was +13 for the game (the bench were all in the plus). OKC's starters all had a negative plus/minus with KD being -9 and Westbrook -11. The starters lost them that game and the 2-0 lead which changed the momentum.
Harden had two inefficient shooting nights in games 3 and 4 (2-10 both times) but basically all three guys were off from three (0-4 for Harden, 1-4 for both KD and Russ) but Russ and KD ate up all the extra shots while Harden facilitated and rebounded. He led the team in rebounding with 10 in one of those games and led the team in assists and was tied for 2nd in rebounds the other game.
He contributed but as Bosh has stated the Heat's game plan was to limit Harden which explains why he couldn't get the easy twos to ramp up his shooting efficiency and overall points. One of those losses KD had 5 TOs in a game decided by six points compared to Harden's 2 TOs.
Beard was reasonably efficient in the elimination game too but Russ was chucking/bricking and KD had 7 TOs to Harden's 3. They got thoroughly outplayed and all had poor plus/minus stats obviously but KD and Russ were the worst of the three (though Harden played fewer minutes).
Because Harden couldn't be the hero third guy he was against the Spurs due to the Heat's game plan, people spun a narrative of him choking when in reality he contributed enough to make some of those losses winnable. The Heat just were the better team with a solid game plan.