It’s one of those cases where they’re not entirely wrong or right. Yes there would be issues with GC, but the delegates handle those cases and there are plenty of static delegates throughout the engine so it shouldn’t be a huge worry.
Hard to say. Is the Spline Manager the only thing that’s going to register with that delegate? The Actor and Manager feel like two parts of a whole and so I wouldn’t see any problem with your ASpline calling into USplineManager directly instead of trying using a delegate as an intermediary. Either way they are coupled, it just depends on which direction that dependency needs to go.