So in essence, styling a component with em can reduce reusability of components more and more as em gets introduced in various places?
The 'em' sizing is inherited by child elements, which can be either a feature or a bug, depending on interactions.
If you're aware of this and design for it it can be a useful feature, and enhances reusability. If you're unaware of this, or your design is complex, it's much more likely to be a bug. "Bug or not?" is entirely dependent on that context.