See Pocket
The main semantic interest (beside performance benefits) is the ability to write
if (entity.hasComponent!Foobar)
entity.component!(Foobar).doThing(...);Also, apply a system only to entities with the relevant components (example?)
Every system has two tiers of coupling: the components it uses, and other systems using the same components.
That architecture is most useful when the data stored by a component has an unambiguous interpretation, and systems mutating a same component are aware of each other.
A difficulty of ECS architectures is cases where the developer wants to establish a non-trivial object graph (example?)
(for performance concerns, see Slot Map data structure)