One drawback, depending on the source of your used oil, is shorter fuel filter life. You could also potentially see accelerated wear on any pump that might come before the filter. There's a fair amount of diesel soot particulate in used diesel oil. Other sources may contain varying amounts of other particulate that would be affecting anything in the fuel system upstream of the fuel filter.
Best to filter it well, or at the very least, let it settle for awhile before putting it in the fuel tank.