Now, to get back to the real topic...
Physics-wise, assuming this nicroburst method allows for essentially infinite Pushing force, drag becomes non-negligable. Drag increases as speed increases (this is why terminal velocity is a thing). Because of this, propelling force must increase for an object's top speed to increase.
Let's say a metal sphere is being pushed (a sphere's orientation does not change its drag efficiency, unlike a with coin). As the Push exerted on the sphere strengthens, the sphere's speed increases, making the force of drag also increase. Because of this, there is a point where the coinshot pushes the sphere with so much force that the force of drag increases so much that he is thrown backwards by the Push (Newton's Third Law). If he is Pushing on an object behind him to brace himself, the coinshot will reach a similar force threshold where that object moves or he crushes himself (Crushing one's self this way is a real consideration in Era 1).
The only way to increase the maximum useable force in this scenario is to replace the coinshot with a crasher (A-steel + F-iron twinborn, aka Wax) or a mistborn burning pewter to use the bracing method.
If an object is already at a speed where drag is the limiting factor, crasing into anything would just add to that force. Crashing into stuff would not be a problem if you are far enough below the maximum force threshold, but, the harder the object is to break through, the further you will neeed to be below the threshold.
As for hitting a person, I seriously doubt something the size of a coin would "disintigrate" someone no matter how fast it is going, but I get the sentiment.
No matter what, maximizing a coinshot's force output with nicrobursts would come down to really precise metal reserves.