So I'm writing a story about humans of the modern world being given various superhuman powers by an unknown entity, which eventually causes society to partially collapse and turn into a bizarre mash-up of post-semi-apocalypse and sword and sorcery adventure (various omnipresent powers render most of modern technology useless in a fight, causing people to fight exclusively with medieval-grade weapons and their various superhuman powers).
Now, one of the various things to result from this is the introduction of fantasy races into the world via giving humans the power to shapeshift, and one of the features I knew I wanted to add to one of the fantasy races in my story was an extra set of arms, since the possibilities that opens up both in combat and in everyday life are fascinating to me. But it occurred to me that most of the places an extra set of arms is usually placed on such races would probably run a serious risk of the arms getting tangled up or bumping into each other. So I spent a while trying to find a way to integrate an extra set of arms in a way that would sound right when described and actually work well in practice.
The idea that resulted is essentially that the four arms would all be mounted on the traditional human shoulder area (with a thicker upper torso to let them fit both shoulder joints side by side), but with more flexible shoulder joints for each and a special, additional, internal shoulder joint that, by a combination of weird biology and straight-up magic, allows them to actually rotate the relative orientation of these shoulder joints to each other so they can shift the positioning of them from back-to-front to up-to-down and even a little further upward, so that they can alter the most practical positioning of the arms for what they want to do and, more notably, if they use both arms on one side to grab a single object, they can swing that object around with two arms as if it was one arm and have the same range of movement that a human would have swinging around one arm. Each arm would have the strength of a human arm, and the internal pivoting shoulder joint would have the 1.5 times the strength of a normal human shoulder joint (keep in mind that most of the characters in this story have their strength further doubled by an omnipresent power as well).
Now to me this sounds like a huge advantage. Even realizing the practical limitations on wielding multiple weapons at once, being able to have 50% to 100% more force behind a blow, carry multiple shields, wield a two-handed weapon with a slight sacrifice to grip strength and still have two more hands, wield a bow and arrow while riding a bike in a world where cars are useless and horses are scarce, and utterly dominate hand-to-hand combat with terrifying grappling techniques and hybrid offensive-defensive stances (keep in mind that bare-fisted combat is much more practical here as everyone's hands, feet, head and neck are all but indestructible) sounds like the advantage they'd have over a normal human would be ridiculous.
In fact it seemed like such a huge advantage that when working out how to balance this race, I concluded that merely having their physiology, their arms combined with their generally slightly stronger and more durable bodies, would have to be balanced by severely reducing the types of magic powers they have access to (to keep humans from becoming the boring jack-of-all-trades race, humans are the only ones who can use all six types of magical abilities in my story, and each race pays for their advantages over humans by losing access to at least some of them, usually just one type). I realize it would be impossible to briefly summarize my design philosophy of the powers, so let's ignore them for now and just assume for the sake of argument that they're designed such that mages and physical fighters are pretty much balanced, all other things being equal, and that fighters have the same number of powers as mages, just generally in more physical abilities.
Am I correct in thinking this would be a huge advantage in combat with medieval-level weaponry and that they'd need to pay a steep price elsewhere to be balanced? Am I overestimating how useful this would be? Am I underestimating it? Or is there some fatal flaw I'm missing with the arm setup I've described that I need to work out?