Man, I love a well-done mob queen. The whole point is that vulnerability isn't the opposite of power; it's the source. A character who's just ruthless gets boring. Think of the best ones, like Kate from 'The Dark Verse' series. She's running a syndicate, making brutal calls, but her vulnerability is her found family of misfits inside the organization. Her power is absolute because she can protect them, and her fear of losing them is her biggest weakness. That duality creates the tension.
It's often shown in private moments, in a way she'd never allow publicly. Maybe she's utterly confident ordering a hit, but her hands shake holding a photo of a dead sibling. Or she's fiercely protective of a younger member, a soft spot that rivals can exploit. The balance comes from letting us see the cracks only under extreme pressure. If she's vulnerable all the time, she's not a queen; if she's never vulnerable, she's a cartoon.
Ultimately, the most compelling part is how she uses her own perceived weaknesses as weapons, turning emotional intelligence into strategic advantage.