Your guess is better than mine when it comes to mixing ECS and MonoBehaviours; I've scrupulously avoided trying to do that. In just ECS, I think the answer would depend on what the events record. If there is a large volume of sequential accumulative events applied to a single entity, I'd probably try to pile them up in a DynamicBuffer of each event type and the process them in a later system. If the events are singular, then maybe they could be enableable components on the entities to which they apply. I'm still figuring out these things too, though, especially compared to those experienced users (e.g. tertle) who have a whole framework for broadcasting and consuming events in serial.