It still seems like a player issue rather than a rules issue. If a feruchemist wants to expend the same effort that it takes to rip a steel door off its hinges or chuck a horse, I can easily see how the same force applied to another living creature would be...messy.
I still feel the system as a whole doesn't contemplate Nudges at that range. As already shown, Nudges in every other instance rarely exceed 10 at the most extreme. But, hey, its cool if its not an issue for you.
If nothing else, this thread provides a good warning to a GM that a starting Feruchemist has a very good chance of one shotting an experienced Mistborn, if they are prepared to hulk out like this and there is a good chance that the Feruchemist will have the charges to do it too.
I believe that overall it still seems balanced. A feruchemist can't keep that kind of effort up for too long, and the excess nudges would be wasted when applied to some extras. And there is still the glass cannon/one-hit-wonder effect.
The Feruchemist doesn't need to keep it up for long. He's only got to win one roll at those Nudges, as its an instant kill, and the Feruchemist has better than average odds of success even aganist an experienced Mistborn. Its actually the better strategy for a Feruchemist in that situation IMO. And players are notorious at doing the right thing
But yeah, against a bunch of extras, I agree with you, the hulk out isn't an issue.
What is the feruchemist going to do? Store up strength before the character is ready to go? "Hang on guys! I know that we need to reach that shipment of Atium before it's loaded on the barges, but I need a few hours to store up the necessary strength for the smackdown!"
And all that time spent at less than normal? "Heh, yeah, I'm literally half the man I normally am, but that is so that I can be twice the man I normally am later!"
Its worth noting that a Feruchemist will get 10 times his rating in charges every short breather, of which there should be one per session. Its 30 times his rating in a long breather which you get one every few sessions. So, there is a good chance, a Feruchemist will have the charges to do exactly what is being contemplated without much inconvenience, before you even get to storing in play.
I feel that this issue is going to come up in lots of Mistborn games. That's cool, but its the one thing I have found that it really pays to have your eyes open about when running the RPG.
I think the balance is all a matter of pacing. Just as you don't want mistborn characters to always have access to never ending vials of pure metals, it is also important to control how often a feruchemist stores up with his/her metalminds. They can also "lose" their metalminds. If a mistborn loses a vial, they can borrow from a friend or take from their enemy. Not so with the feruchemist.
I don't disagree that feruchemists are powerful, but I also don't believe them to be unbalanced, considering the other options in the game.
The pacing in Mistborn is pretty explicit though, even in terms of Mistborn vials, especially once you factor in breathers, resources and props. Also, whilst pacing is an important consideration, I am not sure it should be driven by a concern to restrict the power of one PC. In my experience, if that is not done well, it can actually lead to bad feelings and victimisation. Constantly stealing a Feruchemist's metalminds is a bad solution. Props are noted as being a part of the PC and so you need a good narrative reason to part them from them.
Personally, if this becomes an issue in play, I would prefer just to tackle the problem spot direct with the patch already suggested, than try and mess with the wider pacing of the game.