I have not worked on an MMO before, but recently I had the chance to try out my own custom event system on a small multiplayer game (Unity, PUN2). I had most issues with differentiating which events came from which client. Additionally, I had lots of issues differentiating which events were issued locally. In the end, the code ended up being quite messy. If I were to redo the game, I'd use direct method calls where possible with regular callbacks.
Generally, I found that when using event systems you have to be really careful not to over-use it, even small/single player games. Its super hard to debug when everything is an event - if you go this route, you essentially end up in a situation where everything is "global" and can be reached from anywhere (might as well just go full singleton mode at that point). Additionally, I found it difficult having to deal with event handlers which raise other events, or worse, async events, as then it becomes really hard to ensure the correct order of invocations.
If you plan to use an event system, my advice would be (in Unity):
- Reference and raise events only on root Game Object scripts (e.g., have a root "Actor" script which subscribes/publishes events and communicates with its children via properties/C# events)
- Never subscribe or publish events in regular "child" components
- Use DI/service locator to fetch systems/global things and call them directly when possible from your "Actors"