It's more impressive to be able to instantly accelerate the piece of slime to a very high velocity. Hardening the piece goes without saying because otherwise the drag in the atmosphere would disintegrate it before it got anywhere near the target. It's slime, not metal or other tough material, after all. Still, it's pretty wasteful because slime's density probably isn't that much above water, so the slime bullet doesn't carry all that much kinetic energy.