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1) Getting the umpire to call a strike with it (moving that much, the catcher often receives the darne dthing near the ground after it crosses the plate at the bottom of the strike zone and if you can't get the ump to call close pitches, hitters will just read the upward motion at release and never swing
2) Commanding it at all. Pitches that move that much are notoriously difficult to throw within 12 inches of your target
3) Gopheritis. If you miss up with a curveball, a hitter can play to the natural over-spin and create some monster HRs
4) Arm injuries - especially to young pitchers. To get that much movement, you really have to snap it off hard core. His elbow is at serious risk