Voting machines are black boxes which by nature are incompatible with one fundamental principle of democracy: transparency.

This is a major issue because even if the fraud is unproven, voters can legitimately be concerned about the accuracy of how votes are being counted.

That said, some statistical analysis research have shown a strong probability of votes being flipped by electronic machines in past elections. Here is the original research paper: http://madisonvoices.com/pdffiles/2008_2012_ElectionsResults...