Bidens biggest failures were not undoing a lot of the things Trump did in his first term.
I think it was a false belief in that the US should remain consistent and not back peddle on bad policies. So, instead of working with Iran to restore the JCPoA (that they continued to abide by via the UN) he continued to push sanctions. Similar situation with Venezuela. Hell, he even kept a lot of Trump's awful border policy. He absolutely failed in restoring faith in America, on the world stage, after Trump's first term. He needed to push hard in the opposite direction and have actual material changes that Trump (or any Republican) could not undo again.
Now, one could say this is because they didn't think they would lose again to Trump. But, that, in itself, was also a major failure for Biden/Harris. They had no major material improvements to point to as an achievement.