XFCE uses Xft to configure font hinting, subpixel rendering and DPI, just like GNOME does. However Qt5 applications seem to ignore these settings, which means they don't fit in with the other applications (GTK or Qt4).
The problem is apparently that XDG_CURRENT_DESKTOP=XFCE. When I manually override it with XDG_CURRENT_DESKTOP=GNOME, Qt5 applications do follow Xft settings.
I've attached a screenshot of the same Qt5 application running with XDG_CURRENT_DESKTOP=XFCE and XDG_CURRENT_DESKTOP=GNOME. My XFCE desktop is configured to enable subpixel rendering and GTK/Qt4 applications are honoring this. Qt5 only honors the settings when I override XDG_CURRENT_DESKTOP to GNOME.