I get that, the problem is not about necromancy, it's about how it came into being. Something involving souls would most definitely mean Kindred were involved but the scale of power in this world is actually not all that high. Unless 20,000 people were lined up and freshly slaughtered the Kindred would have to bring in whole clans (yes plural) to conjure that much energy. If they are in the stone and the living can use them then this medium must have been crafted which would cause to to be limited by the confines of the kindreds power, and as I said even our most powerful was hardly a badass.
FMA is great but a lot of what happens there is not only done differently but the scale of power is much higher. Someone as powerful as Roy Mustang would be a godly legend and maybe even heralded as a deity in elven culture. Simply put this is not FMA, this is Rothendust. Scrolling through the character roster very few have access to true magic, even the Espers which are suits of armor filled with pure magical energy are limited, mine being a literal suit of armor and our now MIA S'Kar being someone who basically just burns stuff down, they may not feel physical fatigue but they still have their limits. Just as we use metabolic energy, they use magic energy.