Getting tired of that broken ability. It makes everything like a joke.
I suppose that's why she's the Chosen One. Also, she can't sleep for days so as not to lose control over the enslaved beings.
But it probably feels extra unbalanced because there's no "cost" whatsoever, no cooldown, and visibly it takes the same amount of effort/mana to tame, and subsequently order around, a single goblin and a lvl50 ancient lich.
Introducing an additional downside, like her having to fight with the beings' consciousness at times, or juggling her mana expenditure, or maybe even risk being corrupted by the monsters' souls would feel much more "fair". Then again, we don't know everything just yet.