It is specifically stated that the feruchemist can dump it all at once. and even if burning is instantaneous, if I've stored enough speed, I can have that knife in your neck before the thought processes have even registered what's going to happen and told your body to move.
and, hooray for you for pushing a coin against the thing thrown at you. If it's heavier than you, you're just going to push yourself away.
As for hanging up in the air, a) your atium won't last forever, either, and assuming I'll be using up my resources while I cant' reach you is foolish. b) I can find what metal you're pushing on and move it, probably causing you to crash into things. "Oh, you're pushing on this nail? How about I rip it out with stored strength and toss it over there so you go flying in that other direction."
Allomancers can do quite a lot, but trust me, even when they do stand up to a feruchemist, it won't be nearly so one-sided as you seem to think.