I see where you're coming from, but Kuvira displayed to shoot sharp metals at a rock, splitting it in two... Legend of Korra is broadcasted by Nickelodean, a kid's network, so I doubt they would show a metal spike slicing through a human body; by slicing the rock in half, Kuvira implied that she could do it to a human.
I agree that the amount of moves is too much, "It can only cut through WaterManipulation, IceSpike, Torrent, Surge, Earthblast, RaiseEarth, Airblast, Airshield," I think it should only cut through earth bending abilities, and the only way for an earthbender to block it is for them to metal bend them selves. Logically speaking, a sharp piece of metal being hurled would not cut through wind, the wind would change it's course, so air should be able to block it. The flow of water would have similar affects, although, I don't know if possible, I think the metal blades would be able to break through ice just like they do rock, but anyways... The only way I could see a firebender blocking this is by melting the metal, which seems ridiculous, but it has been done plenty of times (especially in ATLA games, just saying).
OR
This move could be blockable by everything, just like any other basic bending move (Water Manip, Fire Blast, etc), except this projectile moves much, MUCH faster than any other bending, making it pretty hard to block.