The major difference between a true ECS paradigm and MonoBehaviours (EC minus S) is that Systems aren’t local, ie you can select and aggregate various entities

The major difference between a true ECS paradigm and MonoBehaviours (EC minus S) is that Systems aren’t local, ie you can select and aggregate various entities and change their states based on the results of your aggregations. A proper system, for example, would be able to add a new enemy entity every 5 seconds and switch the flag that the level is cleared once the total count of present enemies is 0. In fact, it would be quite trivial to implement it. But in Unity you would need some sort of an architectural solution to bypass inherent limitations of EC-S.