a "battering ram" would have three big problems:
1. the reason axes/hammers are swung in that big circle is mostly to give them range, and more energy (due to longer way to accelerate). Your Fist would be somewhat low-ranged and need an incredible acceleration if you don't want to put it on a rather long shaft, with the need of your bot also being rather long. specially vs. the strong spinners nowadays, range is something you shouldn't forget. And together with the gyroscopic effect maybe the main reason we see anything else than drum spinners.
2. if you miss, all that energy is transferred into YOUR bot. with axes/hammers etc. always having the option to just fly themselves when they miss, you would basically hit the back of your front armor (or anywhere inside your bot, depends on mounting). So... if it is strong enough to do dmg to another bot, you'd have to take really care not to kill your own one.
3.as you already stated, actio = reactio. flippers can transfer extreme energy into the other bot, but that's to one part for your own bot being pressed against the ground. and that's the reason why some hammerbots like beta and chomp experiment with magnets and such to keep them to the ground. in your case, you'd punch the enemy, but also kick yourself backward. would need extreme timing and a lot of grip to counter that with your drive system.

So... while i'd like to see it done, i doubt it would be effective.