I don’t have a particular problem with Harriet Tubman, but I don’t think she is the best choice the Treasury could have made if it wanted to put a woman on our currency.
If it had been left up to me, the woman on our $20 bill would be Admiral Grace Hopper. She was a distinguished military officer, a founder of America’s pre-eminence in computer science, and is a reasonably contemporary example toward whom young women can aspire — at a time when America needs desperately to focus on STEM education. Hopper would have been, by a bunch, a more forward-looking choice.