there are some color/media/print-mode combinations that will create banding. since you are a bit of a newbie (i assume). 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 callibrated 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 postion. 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 senquence 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. so no quasi banding effect.
so you see real banding is only a result of the media compensation being off. it can be carefully callibrated away. the lawnmower effect can only be fixed by using uni. this is an issue that exists in all printers. maybe inksets or heat settings can also play a part. my assumption is banding is both user error and nature of the beast and fixable always.