They are, and that's why sibling and child selectors exist. If you only ever define simple classes with no thought about how they'll work in the overall app then you're in for a bad experience with styles cascading to things they're not meant to affect. If you actually use CSS the way it was designed then you have complete control over exactly what styles are applied to which elements.

A good rule in component based development is to define a class name for the component and use ".unique-component-class-name > .class-for-things-in-this-component" to limit the application of styles to that component. This takes a bit of discipline but it's pretty obvious when you get it wrong.