She supported the terrorist and murderer John Brown. Having her on our currency notes is almost as disgraceful as if they put Brown.
In fact, even if you admire Tubman (as I've asked about a thousand times), can anyone really say with a straight face that Harriet Tubman is such an important historical figure that she deserves to be on our currency? James Madison was the principal author of the Constitution. Other than political correctness, what could possibly justify putting Tubman on the $20 when Madison has no place on our currency (at least not any of the ones in wide circulation - I'm not counting dollar coins which have portraits of virtually every past President).
They can do worse than Tubman...such as a proposal to replace Washington with Malcom X!