for most printers uni is only for certain very high resolution medias and even certain colors can be troublesome because of the lawnmower effect. thermal print heads seem to deal with this better than piezo:
there are some color/media/print-mode combinations that will create banding. the reason uni can do what bi directional can't is two things. think of the the ink dots like bombs and the head carriage like the bomber. in uni the plane only drops bombs in one direction. therefore any calibrated point, will drop bombs perfectly in the feed direction (the media feed is going out). bi requires the printer to coordinate a position for both left and right bombers to land perfectly in the same feed position. this doesn't get rid of or create banding in itself. it gets rid of a blur effect in which say a tiny vertical line needs a crisp vertical edge. the printer also lays down ink colors in a sequence (cmyk, etc). each ink has a slightly different viscosity, dry time, opacity... when the printer goes left the sequence creates the color. when it goes right...the sequence creates the color in reverse, allowing minute difference in the blend of the color. (left yellow beneath cyan, right cyan beneath yellow). in uni, the entire image is only, for instance, yellow beneath cyan, for all passes.
