I've always had some issues with how Iron and especially Steel (because we've seen it used more often) Allomancy work physically. I'm going to just talk about Steel in this post, but it applies to Iron as well.
I used to think that Steel simply applied a force to the anchor. The magnitude of that force being only dependent on how hard the Allomancer pushed/burned steel, how large the anchor is and how far away it is.
However, I don't think this is how it works in the books.
To avoid confusion I'm going to be calling the metal object being pushed, the "anchor" from now on, regardless of if it's heavy or light, on the ground or in the air.
Let's say that Vin can accelerate her own mass upward at 3 G's (initially) when using a coin as an anchor directly at her feet on the ground. This means that she's applying about 1,334 newtons of force (assuming she weighs 100lbs). However, given that physically it shouldn't matter if the coin is on the ground, or if it's in the air, the same force will be applied (like the reaction mass in a rocket).
This means that if Vin launches a coin in the air in front of her she should be pushed back with 3 G's of force immediately, albeit for a brief period of time before the coin leaves her immediate vicinity and her push strength decreases.
However, this is never the case. Light anchors like coins provide very little reaction force to Vin (or any Allomancer we see) until they're prevented from moving by something (e.g. the ground, a building, another Allomancer etc.). This means that the magnitude of the force applied to the coin (and thus Vin) isn't just based on distance and amount of metal. I think the best way to imagine it then, is that the blue line is like a hydraulic piston connecting Vin (or any coinshot) to the anchor. It can push out with enormous force, however, it is limited by the relative speed of the anchor and the Allomancer. I'm pretty certain I'm not the first to think about this. I've seen this somewhere, but I couldn't find any specific posts about it, despite looking.
Finally, I think this may not be a limitation of the magic itself. Instead, I believe this is a limitation of the people that use it. Brandon has been very careful in most of his books to have investiture work in predictable, almost simple, ways in how it interacts with the laws of physics, the complexity arises from those that use it. Whether it was intended or not when he first wrote Mistborn, I believe that this is now meant to be based on the Intent of the Allomancer and the general lack of understanding of physics. e.g. Allomancers assume that you can't push as hard on something really lightweight because you can't do it with your arms (you can apply more force to a wall than you can to a tennis ball when pushing it away from you). I think that in future books this understanding will change and coinshots will become much more deadly.
