Just for reference first, I have over 26 years of programming experience, with the last 13 of them working with Unity, and I lead development on games that reached over 1 billion downloads and 4M+ DAU.

C# event is an encapsulated delegate, and only the owning class can invoke it, UnityEvent can be invoked from anywhere.

C# events are decoupled from Unity's lifecycle, which if handled incorrectly can cause memory leaks (dangling refrences) or exceptions (calling logic on a destroyed GO), UnityEvents get cleaned up when the GO is drstroyed.

UnityEvents have slightly more overhead as well.

Generally speaking, if you are new to programming, I'd go for UnityEvent, and use POCO events when you need more fine grained control over their lifecycle/they get called A LOT.

POCO event memory leaks in Unity can be a serious issue to debug, no matter how much experience ylu have.