The US doesn't feature Harriet Tubman on the $20 because it's full up with racist, slavery-denying fools who still have the mindset of an Andrew Jackson voter and would vote a wheel of racist cheese into the office of president over an eminently qualified black woman any day of any week.
Ironically, Andrew Jackson was reviled at the time. Most Americans were shocked that he got elected, and I'm unsure if he or Woodrow Wilson did more damage to the country.
I would put Nuking Hiroshima and Nagisaki on the list of Truman issues as well.
At the very least Nagisaki. One could argue that the first bomb was necessary after we saw how hard the Japanese fought on Iwo Jima and Okinawa. Japan had already surrendered after Hiroshima, and the only condition they asked was that The Emperor not have to step down. I hindsight I don't think that would have been an issue as he apparently wasn't the driving force behind the war.