Observer & Singleton Patterns

It uses too much memory
It makes the code slower
It can make testing and dependency management more difficult
While the Singleton Pattern is useful in certain scenarios, it can make testing and dependency management more difficult because: 1) It introduces global state which can be harder to mock in tests, 2) It can make dependencies less explicit, 3) It can make it harder to track state changes across an application, and 4) It can violate the Single Responsibility Principle by managing both instance creation and business logic.
It requires complex implementation