In what cases is using a data class not suitable for all classes in a project?

Data classes in Kotlin are convenient for storing data and automatically generate methods like equals(), hashCode(), toString(), and copy(). However, they are not suitable in all cases:

• When the class contains logic: if the class implements complex behavior or business logic, a data class may be inappropriate.
• When inheritance is required: data classes cannot be open for inheritance (by default, they are final).
• If object comparison needs to be controlled: data classes compare all properties, but sometimes more fine-grained comparison is needed.
• For classes with mutable state: data classes are better used for immutable data.

Thus, data classes are suitable for simple DTOs and data models, but not for classes with business logic or complex behavior.