Update April 20, 2017: I'm not sure if I was doing it wrong before or if something was changed/fixed, but reordering classes now work.
The Canvas styleguide indicates that it is based on Flexbox Grid. However, neither .push/.pull classes (like in Bootstrap) nor .first/.last classes seem to work for reordering content when using grid elements. Enabling this would make it easier to design responsive page content. With reordering enabled, mobile users could benefit from consistent vertical ordering of content on small screens, while large screens could rearrange items in multi-column designs for visual variety.